Fault system impact on paleokarst distribution in the Ordovician Yingshan Formation in the central Tarim basin,northwest China

The paleokarst-unconformity at the top of the Ordovician Yingshan Formation in the central Tarim basin was exposed to air for at least 10 Ma, providing favorable conditions for the later formation of high-quality reservoirs. During the karstification process, the fault system plays an important role in controlling the development of paleokarst. This study characterized the fault system through the paleokarst features on the northern slope of the Tazhong High and examined in detail the impacts of the fault system on paleokarst distribution. Formation Micro-Imager logs and seismic curvature change rate were used for characterizing the fault system in different scales. The results revealed three sets of faults in this region. Cross-cutting relationships, unconformities, and relation between faults and karstification indicate Mid-to-Late Ordovician thrust faults, Silurian-Devonian strike-slip faults, and Permian tension faults. “Bright spots” in seismic records calibrated by Formation Micro-Imager logs were used for indicating paleokarst features and different depth distributions, respectively. Furthermore, the study employed spectral decomposition technique to characterize the morphology of paleocave complexes in detail. The results show the Mid-Late Ordovician thrust faults heavily impacted the distribution of paleokarst mainly distributed along master and secondary thrust faults in shallow areas, as well as along master basement-involved thrust faults in deep strata, and along the most pronounced area of paleokarst, Tazhong No. 10 fault zone bounded by back thrusts. The data provides new forecasting information for deep hydrocarbon exploration in paleokarst-related reservoirs of the Yingshan Formation.     

Mesogenetic dissolution of the middle Ordovician limestone in the Tahe oilfield of Tarim basin,NW China

The mesogenetic dissolution is well developed in the middle Ordovician Yijianfang formation (O₂yj) limestone, and the dissolution pores are very important for petroleum accumulation in the south slope area of the Tahe oilfield which lies in the north of the Tarim basin, northwestern China. Mottled, dotted or laminar dissolution can be observed in the O₂yj limestone. Under microscope, the grains, lime matrix and all stages of calcite cements (including oil-inclusion-bearing blocky calcite cements) can all be found dissolved ubiquitously. The stylolites in the limestone were enlarged and rounded because of dissolution. Some dolomite rhombs, precipitated along stylolites in burial environment, were found dissolved as well. The dissolution of the blocky calcite cements and dolomite rhombs and the enlarging of stylolites demonstrate that the dissolution took place in the mesogenetic environment. Concentration of trace elements, including REEs, of the eroded part of the O₂yj limestone is intermediate between that of the uneroded part and that of the underlying lower Ordovician limestone hydrocarbon source rocks. Both δ¹³C_PDB and δ¹⁸O_PDB values of the eroded part are less than those of the uneroded part, respectively. The geochemical characteristics indicate that the eroding fluids are hydrocarbon-bearing fluids coming from the underlying hydrocarbon source rocks.     

Regional structure and tectonic history of the obliquely colliding Columbus foreland basin, offshore Trinidad and Venezuela

Cenozoic eastward migration of the Caribbean plate relative to the South American plate is recorded by an 1100-km-long Venezuela-Trinidad foreland basin which is oldest in western Venezuela (65-55 Ma), of intermediate age in eastern Venezuela (34-20 Ma) and youngest beneath the shelf and slope area of eastern offshore Trinidad (submarine Columbus basin, 15.0 Ma-Recent). In this study of the regional structure, fault families, and chronology of faulting and tectonic events affecting the hydrocarbon-rich Columbus foreland basin of eastern offshore Trinidad, we have integrated approximately 775 km of deep-penetration 2D seismic lines acquired by the 2004 Broadband Ocean-Land Investigations of Venezuela and the Antilles arc Region (BOLIVAR) survey, 325 km of vintage GULFREX seismic data collected by Gulf Oil Company in 1974, and published industry well data that can be tied to some of the seismic reflection lines. Top Cretaceous depth structure maps in the Columbus basin made from integration of all available seismic and well data define for the first time the elongate subsurface geometry of the 11-15 km thick and highly asymmetrical middle Miocene-Recent depocenter of the Columbus basin. The main depocenter located 150-200 km east of Trinidad and now the object of deepwater hydrocarbon exploration is completely filled by shelf and deepwater sediments derived mainly from the Orinoco delta. The submarine Darien ridge exhibits moderate (20-140 m) seafloor relief, forms the steep (12°-24°), northern structural boundary of the Columbus basin, and is known from industry wells to be composed of 0.5-4.5 km thick, folded and thrust-imbricated, hydrocarbon-bearing section of Cretaceous and early Tertiary limestones and clastic rocks. The eastern and southern boundaries of the basin are formed by the gently (1.7°-4.5°), northward-dipping Cretaceous-Paleogene passive margin of South America that is in turn underlain by Precambrian rocks of the Guyana shield.Interpretation of seismic sections tied to wells reveals the following fault chronology: (1) middle Miocene thrusting along the Darien ridge related to highly oblique convergence between the Caribbean plate and the passive margin of northern South America; continuing thrusting and transpression in an oblique foreland basin setting through the early Pleistocene; (2) early Pliocene-recent low-angle normal faults along the top of the Cretaceous passive margin; these faults were triggered by oversteepening related to formation of the downdip, structurally and bathymetrically deeper, and more seaward Columbus basin; large transfer faults with dominantly strike-slip displacements connect gravity-driven normal faults that cluster near the modern shelf-slope break and trend in the downslope direction; to the south no normal faults are present because the top Cretaceous horizon has not been oversteepened as it is adjacent to the foreland basin; (3) early Pliocene-Recent strike-slip faults parallel the trend of the Darien ridge and accommodate present-day plate motions.     

Constraining tectonic compression processes by reservoir pressure evolution: Overpressure generation and evolution in the Kelasu Thrust Belt of Kuqa Foreland Basin,NW China

The Kuqa Foreland Basin (KFB) immediately south of the South Tianshan Mountains is a major hydrocarbon producing basin in west China. The Kelasu Thrust Belt in the basin is the most favorable zone for hydrocarbon accumulations. Widespread overpressures are present in both the Cretaceous and Paleogene reservoirs with pressure coefficients up to 2.1. The tectonic compression process in KFB resulted from the South Tianshan Mountains uplift is examined from the viewpoint of the overpressure generation and evolution in the Kelasu Thrust Belt. The overpressure evolution in the reservoir sandstones were reconstructed through fluid inclusion analysis combined with PVT and basin modeling. Overpressures at present day in the mudstone units in the Kelasu Thrust Belt and reservoir sandstones of the Dabei Gas Field and the Keshen zone are believed to have been generated by horizontal tectonic compression. Both disequilibrium compaction and horizontal tectonic compression are thought to contribute to the overpressure development at present day in the reservoir of the Kela-2 Gas Field with the reservoir sandstones showing anomalously high primary porosities and low densities from wireline log and core data. The overpressure evolution for the Cretaceous reservoir sandstone in the Kelasu Thrust Belt evolved through four stages: a normal hydrostatic pressure (>12–5 Ma), a rapidly increasing overpressure (∼5–3 Ma), an overpressure release (∼3–1.64 Ma) and overpressure preservation (∼1.64–0 Ma). Overpressure developed in the second stage (∼5–3 Ma) was generated by disequilibrium compaction as tectonic compression due to the uplift of the Tianshan Mountains acted at the northern monocline of KFB from 5 Ma to 3 Ma, which provided abundant sediments for the KFB and caused the anomalously high sedimentation rate during the N₂k deposition. From 3 Ma to 1.64 Ma, the action of tectonic compression extended from the northern monocline to the Kelasu Thrust Belt and returned to the northern monocline of KFB from 1.64 Ma to present day. Therefore, the horizontal tectonic compression was the dominant overpressure mechanism for the overpressure generation in the third stage (∼3–1.64 Ma) and overpressure caused by disequilibrium compaction from 5 Ma to 3 Ma was only preserved in the Kela-2 Gas Field until present day.     

Study on the slope break belts in the Jurassic down-warped lacustrine basin in western-margin area, Junggar Basin, northwestern China

The Junggar Basin is one of the large-scale petroliferous basins in northwestern China. During the Jurassic Age it was a down-warped continental basin. Different types of slope break belts with different origins have been identified in the basin, including the fracture, flexure, erosional and depositional slope break belts. Fracture and flexure slope break belts were mainly developed in the western-margin area of the Junggar Basin. Slope break belts can be further divided into two types, which are basin margin and inner basin according to their geographical locations. Tectonic movements are the important origin mechanisms controlling the development of the slope break belts, such as deep-seated thrust structure, inherited paleouplift and secondary fracture movement in the basin. Obviously slope break belts are developed in episodes, developed in multi-stages and are differential in movements and can be inherited and transformed due to the changes of tectonic movements in different periods and different areas. Detailed studies indicate that slope break belts obviously control the strata onlap, the vertical lithologic succession and the vertical and lateral distribution of depositional systems. Slope break belts can control the distribution of different non-structural traps.     

A 1.5D model of the steady-state thermal structure of detachment folds

We present a 1.5D steady-state model of thermal perturbations in the core of detachment anticlines. The model incorporates the effects of heat conduction, advection by folding, radiogenic sources, and conversion of work into heat by viscous dissipation. We work out analytical solutions of model's equations for heat flow and temperature field for the case of low amplitude detachment folds growing in a thick homogeneous stratigraphy.Our analytical solutions for the simplest case of no heat sources show that detachment folds develop a thermal boundary layer. Conduction of heat dominates inside the layer, whereas advection of heat by folding prevails outside the boundary layer. We also find that heat flow in a growing detachment fold decays exponentially with the square of the thickness of the detached stratigraphic section. When heat sources are included into the problem we find that radiogenic heat generates thermal anomalies at the crest of the anticlines whereas viscous dissipation produces thermal anomalies at the core of the detachment fold.We analyze three cases of geological interest: detachment folds growing in evaporitic sequences, marine carbonates, and shaley sediments. For a typical uplift rate of 2 mm/yr conduction dominates in the first class of rocks. Viscous dissipation dominates in folds growing in limestone due to its high viscosity. Advection and the generation of heat by radiogenic isotopes dominate the growth of detachment folds in shale. Additionally, the cooling histories of Lagrangian particles embedded in the fold for the last two cases are analyzed. The core of the fold is characterized by rapid cooling whereas the limbs cool at slower rates than the core. Thus, the development of detachment anticlines may either accelerate maturation of source rocks and expand the maturation window or halt it by removing source rocks from the generating window.     

Discussion for comments of Ehrenberg and Bjørlykke on paper “Formation mechanism of deep Cambrian dolomite reservoirs in the Tarim basin,northwestern China”

In the paper “Formation mechanism of deep Cambrian dolomite reservoirs in the Tarim basin, northwestern China” (Zhu et al., 2015), we concluded that hydrothermal alteration further enhanced porosity in the deep Cambrian dolomite reservoirs in the Tarim Basin, NW China. Professor Ehrenberg and Bjørlykke made comments that the hydrothermal dolomitization and increase in porosity were not well supported and casted doubt on the exploration potential. We insist the influence of hydrothermal alteration on and large exploration potential in the deep Cambrian dolomite reservoirs due to the unique geological conditions and the recent exploration results.     

Evaluation method and application of the relative contribution of marine hydrocarbon source rocks in the Tarim basin: A case study from the Tazhong area

In total, 2.37 million tons of marine crude oil originating from mixed source rocks has been discovered in the Tarim basin. Geological and geochemical analyses have confirmed that these mixed hydrocarbons are mainly from two sets of source rocks, including the Cambrian – Lower Ordovician and Middle-Upper Ordovician hydrocarbon source rocks. In this study, we determined the set of source rocks primarily responsible for the mixed hydrocarbons and the next location to be explored. Differences in n-alkane carbon isotopes in end-member oils from Cambrian–Lower Ordovician and Middle-Upper Ordovician source rocks were examined. A material balance model and simulation methods were used to evaluate the relative amounts contributed by each source. The results from known reserves in the Tazhong area show that the mixing ratio or contribution is up to 65% from Cambrian–Lower Ordovician source rocks and is generally higher than that from Middle-Upper Ordovician source rocks. The discovery of deep hydrocarbons has caused the total oil contribution from the Cambrian–Lower Ordovician to increase. The mixing ratio of Cambrian–Lower Ordovician oil varies depending on the well, formation, and block. It increases from west to east horizontally and from top to bottom vertically. Hydrocarbons from Cambrian–Lower Ordovician source rocks migrate upward along faults, and the mixing ratio decreases as the distance from the oil source fault increases. Favorable areas for Cambrian–Lower Ordovician hydrocarbon exploration are deep layers and areas near the fault zone that are connected to deep layers. The material balance model for carbon isotopes and evaluation methods for relative contributions considered differences in relative concentration and carbon isotope structure of n-alkanes. Herein, new methods for the identification and evaluation of hydrocarbons in the petroleum system of this superimposed basin are presented.     

Organic material accumulation of Carnian mudstones in the North Qiangtang Depression,eastern Tethys: Controlled by the paleoclimate,paleoenvironment, and provenance

The Late Triassic mudstones are considered to be the most significant hydrocarbon source rocks (TOC: 0.54%–3.29%) in the North Qiangtang Depression, eastern Tethys. Here, we present geochemical data from the Woruo Mountain Carnian mudstones, in order to investigate their paleoclimate, paleoenvironment, and provenance and to analyze the mechanism of organic material accumulation. The paleoclimate condition was warm and humid during the Carnian mudstones deposition, as indicated by moderate chemical index of alteration (CIA; 73–76), which may be connected with the Late Triassic Carnian stage global climate event in the Tethys. The low U/Th (0.17–0.25) and C_org/P_tot (7–33) ratio values and moderate manganese contents, reflect the oxidizing conditions during the Carnian mudstones deposition. The relatively high primary productivity in this study is supported by the relatively high P concentrations. The Al₂O₃–(CaO* + Na₂O)–K₂O ternary plot and Th/Sc–Zr/Sc crossplot reflect that the source areas have undergone a medium chemical weathering with weak sedimentary recycling. The TiO₂–Zr, Co/Th–La/Sc, La/Th–Hf, and La/Yb–∑REE bivariate diagrams indicate that the provenance of Carnian mudstones was primarily from felsic igneous rocks. The collision setting has been identified based on the multi-major elements discriminate plots in the present study. The Riwanchaka and Mayigangri masses to the southwest and south of the study area consisting mainly of Middle-Triassic granodiorite and Late-Triassic granite are likely responsible for supplying provenance to the Woruo Mountain Carnian mudstones, which have similar REE patterns. The relatively high TOC contents of Carnian mudstones are related to high paleoproductivity and fast sedimentation rates, which will lead to preservation of some organic matter even when bottom waters are completely oxidizing. The detrital input during the Carnian mudstones deposition would result in dilution of organic matter.     

Geochemical characteristics of the Paleogene shales in the Dongying depression,eastern China

Fluctuations in lacustrine sedimentary environments significantly affect distributions of organic matter (OM), uranium, and other elements in shales. In this study a high-resolution geochemical record of fluctuations in the paleo-depositional environment of a terrestrial lake basin is provided on the basis of extensive samples collected from the Member 3 of the Paleogene Shahejie Formation (Es₃) of the Niu-38 well in the Dongying Depression, Eastern China. These samples were tested for total organic carbon (TOC), element concentrations, and biomarkers to study the evolution and fluctuation in the depositional environments of an ancient lake basin and associated geochemical response. The evolution and fluctuation of the sedimentary environment from a deep lake to a semi-deep lake and then to a shallow lake delta were indicated by geochemical response. During this evolution, the values of TOC, S₁, S₂, Sr, and Ts/(Ts + Tm) remarkably decreased, whereas those of Co, Ni, Rb, Na, Fe/Mn, Fe/(Ca + Mg), and C₂₉ mortane/C₂₉ hopane significantly increased. The deep lake basin shows depositional fluctuations, as indicated by rock lithofacies and their geochemical parameters. A close interrelationship was observed among U concentration, TOC content, and inorganic element content. Uranium concentrations are positively correlated with TOC contents, Ca and Sr concentrations, and Sr/Ba and Ca/Mg ratios but negatively with K, Na, Ba, and Rb contents and Fe/(Ca + Mg) and Fe/Mn ratios. The observed increase in U concentration in the lower Es₃ section is closely related to surface adsorption by clay minerals and OM, together with some replacements of Ca and Sr by U in the shales.     

Stratigraphic evolution of a submarine channel-lobe complex system in a narrow fairway within the Magallanes foreland basin, Cerro Toro Formation, southern Chile

This study integrates newly acquired stratigraphic data, geologic mapping, and paleocurrent data to constrain the stratigraphic evolution of the oldest channel-lobe complex in the Upper Cretaceous Cerro Toro Formation in the Silla Syncline area of the Magallanes Basin, termed the Pehoe member. The Pehoe member ranges in thickness from 60 m in the north to at least 410 m farther down system and comprises three separate divisions (A, B, and C). A lower conglomerate unit and an upper one, termed Pehoe A and C divisions respectively, represent the fill of major incised submarine channels or channel complexes. These are separated by stratified sandstone of the Pehoe B division, representing a weakly confined lobe complex, either transient or terminal.The integration of new data with observations from previous studies reveal that the three main coarse-grained conglomerate and sandstone members in the Cerro Toro Formation in the Silla Syncline include at least seven distinct submarine channels or channel complexes and two major lobe complexes. The thinning and disappearance of these units along the eastern limb of the syncline reflect confinement of the flows to a narrow trough or mini-basin bounded to the east by a topographic high. This confinement resulted in unidirectional paleocurrents to the south and southeast in all deposits. Changes in depositional geometries are interpreted as reflecting changes in sediment supply and relative confinement. Submarine channels were from 700 m to 3.5 km wide and occupied a fairway that was 4-5 km wide. Flows moving south and southeast in this mini-basin probably crossed the eastern topographic high south of the present exposures and joined those moving southward along the axis of the foreland basin at least 16 km to the east.     

Impacts of the tectonic stress field on natural gas migration and accumulation: A case study of the Kuqa Depression in the Tarim Basin,China

Stress, fluid and temperature are three of the major factors that impact natural gas migration and accumulation. In order to study the influences of tectonic stress field on natural gas migration and accumulation in low-permeability rocks, we take the Kuqa Depression as an example and analyze the evolution of the structure and tectonic stress field at first. Then we study the influences of tectonic stress field at different tectonic episodes on fractures and fluid potentials through the numerical simulation method on the section across the KL2 gas field. We summarize two aspects of the impact of the tectonic stress field on natural gas migration and accumulation. Firstly, under the effects of the tectonic stress field, the rock dilation increases with the added stress and strain, and when the shear stress of rock exceeds its shear strength, the shear fractures are well developed. On one hand, the faults which communicate with the hydrocarbon source rocks become the main pathways for natural gas migration. On the other hand, these positions where fractures are well developed near faults can become good reservoirs for natural gas accumulation. Secondly, because fluid potentials decrease in these places near the faults where fractures are well developed, natural gas can migrate rapidly along the faults and accumulates. The impact of tectonic stress fields on natural gas migration and accumulation allows for hydrocarbon migration and accumulation in the low-permeability rocks in an active tectonic compressive setting.     

Complex accumulation and leakage of YC21-1 gas bearing structure in Yanan sag,Qiongdongnan basin,South China Sea

YC21-1 is a gas-bearing structure found within the Yanan sag in the Qiongdongnan Basin, South China Sea. While the structure bears many geological similarities to the nearby YC13-1 gas field, it nevertheless does not contain commercially viable gas volumes. The main reservoirs of the YC21-1 structure contain high overpressures, which is greatly different from those of the YC13-1 structure. The pressure coefficients from drillstem tests, wireline formation tests and mud weights are above 2.1. Based on well-log analysis, illite content and vitrinite reflectance data of mudstones in well YC21-1-2, combining with tectonic and sedimentation characteristics, the timing and causes of overpressure generation are here interpreted. The results indicate the existence of two overpressure segments in the YC21-1 structure. The first overpressure segment resides mainly within the lower and the middle intervals of the Yinggehai Formation, and is interpreted to have been mainly caused by clay diagenesis, while disequilibrium compaction and hydrocarbon generation may also have contributed to overpressure generation. The second overpressure segment comprising the Sanya Formation (Pressure transition zone) and the Lingshui and Yacheng Formations (Hard overpressure zone) is interpreted to owe its presence to kerogen-to-gas cracking. According to petrography, homogenization temperature and salinity of fluid inclusions, two stages of oil-gas charge occurred within the main reservoirs. On the basis of overpressure causes and oil-gas charge history, combining with restored tectonic evolution and fluid inclusion characteristics, a complex accumulation and leakage process in the YC21-1 gas bearing structure has been interpreted. Collective evidence suggests that the first oil charge occurred in the Middle Miocene (circa 16.3–11.2 Ma). Small amount of oil generation and absence of caprocks led to the failure of oil accumulation. Rapid subsidence in the Pliocene and Quaternary gave rise to a sharp increase in geotemperature over a short period of time, leading to prolific gas-generation through pyrolysis and, consequently, overpressure within the main reservoirs (the second overpressure segment). During this period, the second gas charge occurred in the Pliocene and Quaternary (circa 4.5–0.4 Ma). The natural gas migrated in several phases, consisting of free and water soluble phases in a high-pressure environment. Large amounts of free gas are considered to have been consumed due to dissolution within formation water in highly pressured conditions. Water soluble gas could not accumulate in high point of structure. When the pore-fluid pressures in main reservoirs reached the fracture pressure of formation, free gas could leak via opened fractures within cracked caprocks. A repeated fracturing of caprocks may have consumed natural gas stored in formation water and have made water-soluble gas unsaturated. Therefore, the two factors including caprocks fracturing and dissolution of formation water are interpreted to be mainly responsible for the failure of natural gas accumulation in the YC21-1 structure.     

Effect of sedimentary heterogeneities on hydrocarbon accumulations in the Permian Shanxi Formation,Ordos Basin,China: Insight from an integrated stratigraphic forward and petroleum system modelling

Sedimentary heterogeneities are ubiquitous in nature and occur over a range of scales from core, reservoir to basin scales. They may thus exert significant influences on hydrocarbon generation, migration and accumulation. The sedimentary heterogeneities of the Permian Shanxi Formation in the Ordos Basin, China were modelled using Sedsim, a stratigraphic forward modelling program. The simulation results were then used to construct a 3D petroleum system model using PetroMod. The effects of sedimentary heterogeneities on hydrocarbon accumulations were evaluated by comparing the integrated Sedsim-PetroMod model with the classic 3D basin model. The Sedsim simulation shows that considerable sedimentary heterogeneities are present within the Shanxi Formation, as a result of the interplay of the initial topography, tectonic subsidence, base level change and sediment inputs. A variety of lithologies were developed both laterally and vertically within the Shanxi Formation at kilometre and metre scales, respectively, with mudstones mainly developed in the depositional centre, while sandstones developed in the southern and northern margin areas. A typical source-ward retrogradation is well developed within the Lower Shanxi Formation.A base-case classic 3D basin model was constructed to quantify the Permian petroleum system in the Ordos Basin. The geological and thermal models were calibrated using Vr and borehole temperature data. The source rocks of the Upper Paleozoic became mature (R_o > 0.5%) and high mature (R_o > 1.2%) in the late Triassic and late Jurassic, respectively, in the central and southern areas. During the Early Cretaceous, a tectonically induced geothermal event occurred in the southern Ordos Basin. This caused the source rocks to reach over maturity (R_o > 2.0%) quite rapidly in the early Late Cretaceous in the central and southern areas. All the source rock transformation ratios (TR) at present are greater than 70% in the P₁ coal and P₁ mudstone layers with TR values approaching 100% in the central and southern areas. The transformation ratios of the P₁ limestone are close to 100% over the entire interval.In the base-case model, a large amount of hydrocarbons appear to have been expelled and migrated into the Shanxi Formation, but only a minor amount was accumulated to form reservoirs. In the model, the Shanxi Formation sandstone layer was set to be homogeneous vertically and there was no regional seal rocks present at the top of the Shanxi Formation. Therefore hydrocarbons could not be trapped effectively with only minor accumulations in some local structural highs where hydrocarbons are trapped both at the top and in the up-dip direction by the adjacent mudstone facies. In contrast, the integrated Sedsim-PetroMod model takes into account of the internal lithological and sedimentary facies heterogeneities within the Shanxi Formation, forming complex contiguous sandstone-mudstone stacking patterns. Hydrocarbons were found to have accumulated in multiple intervals of lithological traps within the Shanxi Formation. The results indicate that lithological distinctions, controlled by sedimentary heterogeneities in three dimensions can provide effective sealing in both the top and up-dip directions for hydrocarbon accumulations, with gas being mainly accumulated near the depocentre where lithological traps usually formed due to frequent oscillations of the lake level.     

In situ stress field evaluation of deep marine tight sandstone oil reservoir: A case study of Silurian strata in northern Tazhong area,Tarim Basin,NW China

Deep marine tight sandstone oil reservoirs are the subject of considerable research around the world. This type of reservoir is difficult to develop due to its low porosity, low permeability, strong heterogeneity and anisotropy. A marine tight sandstone oil reservoir is present in the Silurian strata in the northern Tazhong area of the Tarim Basin, NW China, at a depth of more than 5000 m. The porosity is between 6% and 8%, and the gas permeability is between 0.1 and 1 × 10⁻³ μm². The features of this type of reservoir include the poor effects of conventional fracturing modifications and horizontal wells, which can lead to stable and low levels of production after staged fracturing. Here, we conduct a comprehensive evaluation of the mechanical properties of the rock and the in situ stress of the target tight sandstones by using numerous mechanical and acoustic property tests, conducing crustal stress analysis and using data from thin section observations. The dispersion correction technique is used to transform velocity at the experimental high frequency (1 MHz) to velocity at the logging frequency (20 kHz). The logging interpretation models of the transverse wave offset time, mechanical parameters and in situ stress are calculated, and each model represents a high precision prediction. Simulating the in situ stress field of the Silurian strata using a three-dimensional finite element method demonstrates that the average error between the simulation result and the measured value is less than 6%. The planar distribution of each principal stress is mainly controlled by the burial depth and fault distribution. By conducting in situ stress orientation analysis for the target layer via the analysis of paleomagnetism, borehole enlargement, fast shear wave orientation and stress field simulation, we show that the direction of the maximum horizontal stress is N45E. In this paper, a typical and successful comprehensive evaluation of the stress field of the deep tight sandstone oil reservoir is provided.     

Impact of basin structure and evaporite distribution on salt tectonics in the Algarve Basin,Southwest Iberian margin

The SW Iberian margin developed as a passive margin during Mesozoic times and was later inverted during the mainly Cenozoic Alpine orogeny. The initial syn-rift deposits include a Lower Jurassic evaporite unit of variable thickness. In the onshore, this unit is observed to thicken basinward (i.e., southward), in fault-controlled depocenters, and salt-related structures are only present in areas of thick initial evaporites. In the offshore, multiple salt-structures cored by the Lower Jurassic evaporites are interpreted on seismic reflection data and from exploratory drilling. Offshore salt structures include the allochthonous Esperança Salt Nappe, which extends over an area roughly 40 × 60 km. The abundance of salt-related structures and their geometry is observed to be controlled by the distribution of evaporite facies, which is in turn controlled by the structure of rift-related faulting. This paper presents a comprehensive study of salt tectonics over the entire onshore and offshore SW Iberian passive margin (southern Portugal and Gulf of Cadiz), covering all aspects from initial evaporite composition and thickness to the evolution of salt-related structures through Mesozoic extension and Cenozoic basin inversion.     

Effects of structural segmentation and faulting on carbonate reservoir properties: A case study from the Central Uplift of the Tarim Basin,China

The largest condensate field of the Ordovician reef-shoal reservoirs, in China, was discovered along the N°1 fault zone in the Tarim Basin. However, current drilling activities are hindered by the low and unstable production from these carbonate reservoirs. A detailed structural analysis of 3-D seismic data identified three segments along the N°1 fault zone characterized by differences in structural geometry and evolution. Segmentation of the N°1 fault zone has resulted in the variation of characteristics and distribution of the Upper Ordovician carbonate reservoirs. Due to fault induced differential uplift before the Late Ordovician, a narrow and thick-rimmed platform margin rich in reefs has developed in the eastern segment, whereas wider and thinner shoals developed in the central and western segments. We clearly identified a fault block uplift related erosional unconformity, and associated karstification at the top of the Upper Ordovician carbonates in the eastern segment. In this segment, the karst caves resulted in the high and stable hydrocarbon production. In the central segment, good matrix reservoirs exist, but due to the lack of fault-related enhancement of the reservoirs quality, only relatively low production is observed. In the western segment, where fracture-vug reservoirs occurred in relation to later fault damage zones, production is much greater but unstable. Large-scale fracture-caves reservoirs related to fault activity, rather than the good unfaulted matrix reservoirs, lead to the high production of oil and gas. We classify these reef-shoal reservoirs as faulting modified reservoirs. Further exploitation should focus more on the fault damage zone beyond the platform margin rather than the microfacies in the Lower Paleozoic carbonates.     

南海深海盆表层沉积物氮的地球化学特征与生态学功能

研究了南海深海盆区域(南沙海槽西南部)表层沉积物中氮的形态、分布及其在生物地球化学循环中的功能.研究表明,表层沉积物中不同形态氮的含量不同.其中,氧化还原转化态的氮(SOEF-N)含量最高,平均为68.3μg/g,占总氮(TN)的7.08%;弱酸转化态氮(WAEF-N)含量最小,仅占总氮(TN)的1.09%.离子交换态(IEF-N),WAEF-N,S OEF-N及TN的地球化学分布特征存在一定相似性:均由海槽东西两侧向中央递增,并在槽底呈高含量分布;SAEF-N(强碱转化态)分布则与该趋势相反.IEF-N,SAEF-N和SOEF-N的分布主要受沉积物中有机碳含量(OC)控制;而WAEF-N则与碳酸盐(CaCO₃)存在显著的负相关关系;TN与OC不具有显著意义的相关,间接说明二者来源的不同.同时,各形态氮的分布还与沉积物粒度类型密切联系.此外,研究区域内由沉积物提供的氮源很大程度上补偿了浮游植物对水体中营养盐的消耗,对维持该海域的初级生产力水平起到一定作用.其中,IEF-N和SOEF-N的释放对浮游植物生长及初级生产力的贡献较为显著.     

Utility of GOI,QGF, and QGF-E for interpreting reservoir geohistory and oil remigration in the Hudson oilfield,Tarim basin,northwest China

The physical mechanisms responsible for hydrocarbon migration in carrier beds are well understood. However, secondary migration is one of poorly understood facets in petroleum system. The Carboniferous Donghe sandstone reservoir in the Tarim Basin's Hudson oilfield is an example of a secondary (or unsteady) reservoir; that is, oil in this reservoir is in the process of remigration, making it a suitable geologic system for studying hydrocarbon remigration in carrier beds. Experimental methods including grains containing oil inclusions (GOI), quantitative grain fluorescence (QGF) and quantitative grain fluorescence on extract (QGF-E) -- together with the results from drilling, logging and testing data -- were used to characterize the nature of oil remigration in the Donghe sandstone. The results show that (1) significant differences exist between paleo- and current-oil reservoirs in the Donghe sandstone, which implies that oil has remigrated a significant distance following primary accumulation; (2) due to tectonic inversion, oil remigration is slowly driven by buoyancy force, but the oil has not entered into the trap entirely because of the weak driving force. Oil scarcely enters into the interlayers, where the resistance is relatively large; (3) the oil-remigration pathway, located in the upper part of the Donghe sandstone, is planar in nature and oil moving along this pathway is primarily distributed in those areas of the sandstone having suitable properties. Residual oil is also present in the paleo-oil reservoirs, which results in their abnormal QGF-E. A better understanding of the characteristics of oil remigration in the Donghe sandstone in the Hudson oilfield can contribute to more effective oil exploration and development in the study area.     

Relationship between salt diapirism and faulting in the central structural belt of the Dongying sag, Bohai Gulf basin, China

1 Introduction D iapiric structures are an im portant andw idespread structural style in m any tectonic settings(Ism ail-Zadeh etal.,2001),including m agm a diapirs,salt diapirs, m ud diapirs, and serpentinite diapirs.C om pared w ith the form ertw o,m ud diapirism in sedi-m ents is rarely reported and studied although it hasbeen know n for a long tim e both on land and underthe sea.H ow ever,w ith the geologicalinvestigations ofO D P and related on-land studies of accretionaryprism s, m ud…