Thermal history and source rock characterization of a Paleozoic section in the Awbari Trough,Murzuq Basin,SW Libya

In some areas of the Murzuq Basin, SW Libya, the lower Silurian member comprises highly radioactive shales (Hot Shale), which are regarded as the most significant factor controlling petroleum generation in the basin. In this context, it was the goal of our project to study the distribution and maturity of the Hot Shale in the framework of basin evolution. Organic geochemical, organic petrological and basin modelling methods were used to obtain a more thorough understanding. Four wells from the northern and central part of the Awbari Trough have been selected for this study.     

Stable carbon isotopic ratios of CH4-CO2-bearing fluid inclusions in fracture-fill mineralization from the Lower Saxony Basin (Germany) - A tool for tracing gas sources and maturity

The stable carbon isotopic ratios (δ¹³C) of methane (CH₄) and carbon dioxide (CO₂) of gas-rich fluid inclusions hosted in fracture-fill mineralization from the southern part of the Lower Saxony Basin, Germany have been measured online using a crushing device interfaced to an isotopic ratio mass spectrometer (IRMS). The data reveal that CH₄ trapped in inclusions seems to be derived from different source rocks with different organic matter types. The δ¹³C values of CH₄ in inclusions in quartz hosted by Carboniferous rocks range between −25 and −19‰, suggesting high-maturity coals as the source of methane. Methane in fluid inclusions in minerals hosted by Mesozoic strata has more negative carbon isotope ratios (−45 to −31‰) and appears to represent primary cracking products from type II kerogens, i.e., marine shales. There is a positive correlation between increasing homogenization temperatures of aqueous fluid inclusions and less negative δ¹³C(CH₄) values of in co-genetic gas inclusions probably indicating different mtaturity of the potential source rocks at the time the fluids were released. The CO₂ isotopic composition of CH₄-CO₂-bearing inclusions shows slight negative or even positive δ¹³C values indicating an inorganic source (e.g., water-rock interaction and dissolution of detrital, marine calcite) for CO₂ in inclusions. We conclude that the δ¹³C isotopic ratios of CH₄-CO₂-bearing fluid inclusions can be used to trace migration pathways, sources of gases, and alteration processes. Furthermore, the δ¹³C values of methane can be used to estimate the maturity of the rocks from which it was sourced. Results presented here are further supported by organic geochemical analysis of surface bitumens which coexist with the gas inclusion-rich fracture-fill mineralization and confirm the isotopic interpretations with respect to fluid source, type and maturity.     

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.     

低勘探程度盆地模拟研究——以南黄海盆地北部坳陷为例

盆地模拟已成为当前沉积盆地研究的重要工具。南黄海盆地北部坳陷自裂陷期演化以来沉积了巨厚的中-新生代碎屑沉积,近年来的地质调查获取的数据为其盆地模拟研究提供了条件,本次研究在收集相关基础数据的基础上,首先对盆地构造热演化史进行了模拟,重建了盆地热史,模拟结果显示其古热流在中-晚侏罗世平均值约为61mW/m²,在约145-74Ma间不断上升至约80 mW/m²,随后缓慢下降至65 mW/m²,并持续到渐新世末期,据此将盆地演化阶段划分为裂前期、裂陷期及裂后期。盆地模拟结果显示北部坳陷在白垩纪逐步进入强裂陷演化阶段并经历快速沉积过程,至晚白垩纪裂陷发育程度中等,在此基础上,对研究区进行了三维盆地模拟,结果显示北部坳陷生烃门限深度大致位于古近系阜宁组顶部,下伏的侏罗系及白垩系烃源岩基本完成生排烃过程,其中侏罗系烃源岩生排烃主要发生在盆地发育的裂陷期及裂后期,而白垩系及古近系烃源岩生排烃主要发生在裂后期。尽管研究区尚处在低勘探程度阶段,但盆地模拟结果已能为研究区下一步的勘探提供重要的信息,此外,本次研究对模拟过程中的主要不确定性也进行了分析。     

A basin modeling and organic geochemistry study in the Vallecitos syncline,San Joaquin Basin,California

The Vallecitos syncline is a westerly structural extension of the San Joaquin Basin. The Vallecitos oil field, comprised of eight separate areas that produce from Cretaceous and Paleogene reservoirs, accounted for 5.4 MMB of oil and 5.6 BCF associated of gas through 2010. However, exploration for oil and gas in the Vallecitos area is challenging due to structural complexity and limited data. The purpose of this study is to evaluate whether source rocks are actively generating petroleum in the Vallecitos syncline and to improve our understanding of burial history and timing of hydrocarbon generation. We conducted biomarker analysis on twenty-two oil samples from the Vallecitos syncline. Source-related biomarkers show two genetic groups of oil, which originated from two different source rocks. These results differ from earlier published interpretations in which the Kreyenhagen Formation is the only source rock in the Vallecitos syncline, and suggest that the Cretaceous Moreno Formation in the syncline also is an active source rock.Stratigraphic evidence and modeling suggest that late Cenozoic episodes of erosion due to folding and uplift removed significant overburden on the flanks of the syncline. To better understand the petroleum systems and clarify the total active source rocks in the area, 2D burial histories were generated through the Vallecitos syncline. A published cross-section through the deepest part of the syncline was selected to conduct thermal history, basin evolution, and migration analyses. The 2D model results indicate that the lower Kreyenhagen Formation has various maturities within the formation at different locations in the present-day syncline. The basal part of the Kreyenhagen Formation is in the dry gas window and maturity decreases away from the central part to the flanks. It remains immature along shallow portions of the present-day flanks. In contrast, the basal part of the Moreno Formation achieved extremely high maturity (past the gas generation zone) but is in the oil generation zone on the flanks of the syncline at shallow depth. All of our geochemical and 2D model results suggest that there are two active source rocks in the Vallecitos syncline. Accordingly, we propose that there are two active petroleum systems in the Vallecitos syncline.     

Improved understanding of petroleum migration history in the Hongche fault zone,northwestern Junggar Basin (northwest China): Constrained by vein-calcite fluid inclusions and trace elements

Calcite veins and cements occur widely in Carboniferous and Permian reservoirs of the Hongche fault zone, northwestern Junggar Basin in northwest China. The calcites were investigated by fluid inclusion and trace-element analyses, providing an improved understanding of the petroleum migration history. It is indicated that the Hongche fault behaved as a migration pathway before the Early Cretaceous, allowing two oil charges to migrate into the hanging-wall, fault-core and footwall reservoirs across the fault. Since the Late Cretaceous, the Hongche fault has been sealed. As a consequence, meteoric water flowed down only into the hanging-wall and fault-core reservoirs. The meteoric-water incursion is likely an important cause for degradation of reservoir oils. In contrast, the footwall reservoirs received gas charge (the third hydrocarbon event) following the Late Cretaceous. This helps explain the distribution of petroleum across the fault. This study provides an example of how a fault may evolve as pathway and seal over time, and how reservoir diagenetic minerals can provide clues to complex petroleum migration histories.     

Nanoscale pore structure and fractal characteristics of a marine-continental transitional shale: A case study from the lower Permian Shanxi Shale in the southeastern Ordos Basin,China

Shale samples collected from seven wells in the southeastern Ordos Basin were tested to investigate quantitatively the pore structure and fractal characteristics of the Lower Permian Shanxi Shale, which was deposited in a marine-continental transitional (hereinafter referred to as the transitional) environment. Low-pressure nitrogen adsorption data show that the Shanxi Shale exhibits considerably much lower surface area (SA) and pore volume (PV) in the range of 0.6–1.3 m²/g and 0.25–0.9 ml/100 g, respectively. Type III kerogen abundant in the transitional Shanxi Shale were observed to be poorly developed in the organic pores in spite of being highly mature, which resulted in a small contribution of organic matter (OM) to the SA and PV. Instead, I/S (illite-smectite mixed clay) together with illite jointly contributed mostly to the SA and PV as a result of the large amount of inter-layer pores associated with them, which were evident in broad-ion-beam (BIB) imaging and statistical analysis. Additionally, the Shanxi Shale has fractal geometries of both pore surface and pore structure, with the pore surface fractal dimension (D1) ranging from 2.16 to 2.42 and the pore structure fractal dimension (D2) ranging from 2.49 to 2.68, respectively. The D1 values denote a pore surface irregularity increase with an increase in I/S and illite content attributed to their more irregular pore surface compared with other mineralogical compositions and OM. The fractal dimension D2 characterizing the pore structure complexity is closely related to the pore arrangement and connectivity, and I/S and illite decrease the D2 when their contents increase due to the incremental ordering degree and connectivity of I/S- or illite-hosted pores. Meanwhile, other shale constituents (including kaolinite, chlorite, and OM) that possess few pores can significantly increase the pore structure complexity by way of pore-blocking.     

Tracing of deeply-buried source rock: A case study of the WC9-2 petroleum pool in the Pearl River Mouth Basin, South China Sea

The identification of a deeply-buried petroleum-source rock, owing to the difficulty in sample collection, has become a difficult task for establishing its relationship with discovered petroleum pools and evaluating its exploration potential in a petroleum-bearing basin. This paper proposes an approach to trace a deeply-buried source rock. The essential points include: determination of the petroleum-charging time of a reservoir, reconstruction of the petroleum generation history of its possible source rocks, establishment of the spatial connection between the source rocks and the reservoir over its geological history, identification of its effective source rock and the petroleum system from source to trap, and evaluation of petroleum potential from the deeply-buried source rock. A case study of the W9-2 petroleum pool in the Wenchang A sag of the Pearl River Mouth Basin, South China Sea was conducted using this approach. The W9-2 reservoir produces condensate oil and gas, sourced from deeply-buried source rocks. The reservoir consists of a few sets of sandstone in the Zhuhai Formation, and the possible source rocks include an early Oligocene Enping Formation mudstone and a late Eocene Wenchang Formation mudstone, with a current burial depth from 5000 to 9000 m. The fluid inclusion data from the reservoir rock indicate the oil and the gas charged the reservoir about 18–3.5 Ma and after 4.5 Ma, respectively. The kinetic modeling results show that the main stages of oil generation of the Wenchang mudstone and the Enping mudstone occurred during 28–20 Ma and 20–12 Ma, respectively, and that the δ¹³C₁ value of the gas generated from the Enping mudstone has a better match with that of the reservoir gas than the gas from the Wenchang mudstone. Results from a 2D basin modeling further indicate that the petroleum from the Enping mudstone migrated upward along the well-developed syn-sedimentary faults in the central area of the sag into the reservoir, but that the petroleum from the Wenchang mudstone migrated laterally first toward the marginal faults of the sag and then migrated upward along the faults into shallow strata. The present results suggest that the trap structure in the central area of the sag is a favorable place for the accumulation of the Enping mudstone-derived petroleum, and that the Wenchang mudstone-derived petroleum would have a contribution to the structures along the deep faults as well as in the uplifted area around the sag.     

Fractures in the low porosity and ultra-low permeability glutenite reservoirs: A case study of the late Eocene Hetaoyuan formation in the Anpeng Oilfield,Nanxiang Basin,China

The Naxiang Basin, located within the Qin-and-Dabie Mountains orogenic belt in central China, is a small intermountain faulted basin, in which the late Eocene Hetaoyuan Formation in the Anpeng Oilfield is the low porosity and ultra-low permeability glutenite reservoirs. The large-scale fractures are mainly developed in the thick-bedded conglomerates and gravel-bearing gritstones, the small-scale jointed-fractures bounded by layer are mainly developed in the medium-granular and fine sandstones between the conglomerates or gravel-bearing gritstones. There also developed three kinds of micro-fractures, namely intergranular fractures, intragranular fractures and transgranular fractures, in the tight glutenites. The fractures in the glutenite reservoirs are of chiefly high dip-angles. They assume chiefly the E-W strikes, next are the NE-SW and NW-SE strikes. On the log curve, fractures usually show such responses as a decrease of the deep and shallow lateral resistivities, an increase of the acoustic time difference, a reduction of the density, an increase of the compensation neutrons, an occurrence of middle and high gamma ray and an expansion of the calipers. Fractures make contributions to the low porosity and ultra-low permeability glutenite reservoirs by improving the permeability and then porosity. Micro-fractures are the important channels connecting intergranular and intragranular solution pores, thereby making better the connectivity within the tight glutenite reservoirs. The small- and medium-scale fractures serve as the flow channels within the layers of fine and medium-granular sandstones, while the large-scale fractures serve as the main flow channels for the whole reservoir. Under the impact of the present-day stress, the NE-SW fractures perpendicular to the minimum principal stress assume a tensile state and have good connectivity, large apertures and high permeability, and are the major direction for fluid flow, thereby providing a major basis for a further development.     

Modeling an atypical petroleum system: A case study of hydrocarbon generation,migration and accumulation related to igneous intrusions in the Neuquen Basin,Argentina

In the Altiplanicie del Payún area (Neuquen Basin, Argentina), immature source rock sections intruded by up to 600 m thick Tertiary laccoliths show full spectrum maturity aureoles over hundreds of meters from the contacts. Commercial oil accumulations (20–33°API) and oil shows are located along the entire column, both in sandstone/carbonate and fractured igneous reservoirs. A challenging numerical model that included the emplacement of the intrusive bodies, with extreme temperature ranges and unusually short calculation time steps, has been done with the aim to better understand hydrocarbon generation and migration processes related to these thermal anomalies.     

Pore characteristic analysis of a lacustrine shale: A case study in the Ordos Basin,NW China

Organic shales deposited in a continental environment are well developed in the Ordos Basin, NW China, which is rich in hydrocarbons. However, previous research concerning shales has predominantly focused on marine shales and barely on continental shales. In this study, geochemical and mineralogical analyses, high-pressure mercury intrusion and low-pressure adsorption were performed on 18 continental shale samples obtained from a currently active shale gas play, the Chang 7 member of Yanchang Formation in the Ordos Basin. A comparison of all these techniques is provided for characterizing the complex pore structure of continental shales.Geochemical analysis reveals total organic carbon (TOC) values ranging from 0.47% to 11.44%, indicating that there is abundant organic matter (OM) in the study area. Kerogen analysis shows vitrinite reflectance (Ro) of 0.68%–1.02%, indicating that kerogen is at a mature oil generation stage. X-ray diffraction mineralogy (XRD) analysis indicates that the dominant mineral constituents of shale samples are clay minerals (which mainly consist of illite, chlorite, kaolinite, and negligible amounts of montmorillonite), quartz and feldspar, followed by low carbonate content. All-scale pore size analysis indicates that the pore size distribution (PSD) of shale pores is mainly from 0.3 to 60 nm. Note that accuracy of all-scale PSD analysis decreases for pores less than 0.3 nm and more than 10 μm. Experimental analysis indicates that mesopores (2–50 nm) are dominant in continental shales, followed by micropores (<2 nm) and macropores (50 nm–10 μm). Mesopores have the largest contribution to pore volume (PV) and specific surface area (SSA). In addition, plate- and sheet-shaped pores are dominant with poor connectivity, followed by hybrid pores. Results of research on factors controlling pore structure development show that it is principally controlled by clay mineral contents and Ro, and this is different from marine systems. This study has important significance in gaining a comprehensive understanding of continental shale pore structure and the shale gas storage–seepage mechanism.     

Combining rate-controlled porosimetry and NMR to probe full-range pore throat structures and their evolution features in tight sands: A case study in the Songliao Basin,China

To better understand reservoir quality and to produce accurate petrophysical interpretations, it is necessary to understand complex and heterogeneous pore throat structures in tight sands and to develop a technique to reveal the full range of pore and throat distributions. In this study, in order to characterize the features and evolutions of pore throat structures, nine samples from the Lower Cretaceous tight gas sandstone in the Songliao Basin of China are measured by nuclear magnetic resonance (NMR), rate-controlled porosimetry (RCP) and scanning electron microscopy (SEM). Throats with bifractal structures are found in these tight sands and can be divided into a backbone formation (BF) region and a percolation region using RCP data. Because (i) throats in the percolation region record treelike pore structures and are predominant in small pore spaces and (ii) a good correlation exists between NMR-derived T₂ relaxation times and the RCP-derived radius of throats, the throat distribution obtained via RCP can be used to calibrate the NMR PSD and then to partition the PSD into distributions of pore bodies and throats. These data indicate that (i) throats are more common than pore bodies in pore spaces of tight sands with lower permeability, (ii) pore bodies are connected to throats from both the BF and percolation regions, in which the fluid mobility in pore bodies is mainly controlled by the pore to throat ratio (PTR), which is related to the intersection throat of these two regions, and (iii) compaction, dissolution, clay cementation and sediment properties (e.g., rock compositions) have different impacts on the evolutions of pore bodies and throats, in which larger PTR values in tight sands are mainly produced by compaction and pore-bridging clay cementation, and lower contents of pore bodies are commonly related to abundant plastic compositions and pore-bridging clay cementation.     

Dissolution mechanism of a deep-buried sandstone reservoir in a deep water area: A case study from Baiyun Sag,Zhujiang River (Pearl River) Mouth Basin

Dissolution mechanism and favorable reservoir distribution prediction are the key problems restricting oil and gas exploration in deep-buried layers. In this paper, the Enping Formation and Zhuhai Formation in Baiyun Sag of South China Sea was taken as a target. Based on the thin section, scanning electron microscopy, X-ray diffraction, porosity/permeability measurement, and mercury injection, influencing factors of dissolution were examined, and a dissolution model was established. Further, high-quality reservoirs were predicted temporally and spatially. The results show that dissolved pores constituted the main space of the Paleogene sandstone reservoir. Dissolution primarily occurred in the coarse- and medium-grained sandstones in the subaerial and subaqueous distributary channels, while dissolution was limited in fine-grained sandstones and inequigranular sandstones. The main dissolved minerals were feldspar, tuffaceous matrix, and diagenetic cement. Kaolinization of feldspar and illitization of kaolinite are the main dissolution pathways, but they occur at various depths and temperatures with different geothermal gradients. Dissolution is controlled by four factors, in terms of depositional facies, source rock evolution, overpressure, and fault activities, which co-acted at the period of 23.8–13.8 Ma, and resulted into strong dissolution. Additionally, based on these factors, high-quality reservoirs of the Enping and Zhuhai formations are predicted in the northern slope, southwestern step zone, and Liuhua uplift in the Baiyun Sag.     

Simulation of paleotectonic stress fields and quantitative prediction of multi-period fractures in shale reservoirs: A case study of the Niutitang Formation in the Lower Cambrian in the Cen'gong block,South China

Reservoirs where tectonic fractures significantly impact fluid flow are widespread. Industrial-level shale gas production has been established from the Lower Cambrian Niutitang Formation in the Cen'gong block, South China; the practice of exploration and development of shale gas in the Cen'gong block shows that the abundance of gas in different layers and wells is closely related to the degree of development of fractures. In this study, the data obtained from outcrop, cores, and logs were used to determine the developmental characteristics of such tectonic fractures. By doing an analysis of structural evolution, acoustic emission, burial history, logging evaluation, seismic inversion, and rock mechanics tests, 3-D heterogeneous geomechanical models were established by using a finite element method (FEM) stress analysis approach to simulate paleotectonic stress fields during the Late Hercynian—Early Indo-Chinese and Middle-Late Yanshanian periods. The effects of faulting, folding, and variations of mechanical parameters on the development of fractures could then be identified. A fracture density calculation model was established to determine the quantitative development of fractures in different stages and layers. Favorable areas for shale gas exploration were determined by examining the relationship between fracture density and gas content of three wells. The simulation results indicate the magnitude of minimum principal stress during the Late Hercynian — Early Indo-Chinese period within the Cen'gong block is −100 ∼ −110 MPa with a direction of SE-NW (140°–320°), and the magnitude of the maximum principal stress during the Middle-Late Yanshanian period within the Cen'gong block is 150–170 MPa with a direction of NNW-SSE (345°–165°). During the Late Hercynian — Early Indo-Chinese period, the mechanical parameters and faults play an important role in the development of fractures, and fractures at the downthrown side of the fault are more developed than those at the uplifted side; folding plays an important role in the development of fractures in the Middle-Late Yanshanian period, and faulting is a secondary control. This 3-D heterogeneous geomechanical modelling method and fracture density calculation modelling are not only significant for prediction of shale fractures in complex structural areas, but also have a practical significance for the prediction of other reservoir fractures.     

Seismic geomorphology and depositional system of delta and terminal fan: A case study of the Neogene Shawan Formation in the Chepaizi Uplift,Junggar Basin,China

The mainpurpose of this article is to demonstrate the utility of stratal slice images for exploring the sequence stratigraphy and sedimentology of complex depositional systems. A seismic sedimentological study was performed to map sediment dispersal characteristics of the Neogene Shawan Formation in the Chepaizi Uplift of the Junggar Basin, China. The Chepaizi Uplift is developed on the Carboniferous igneous rock basement that lies at the western boundary of the Junggar Basin. The data sources primarily include lithology, well-logging and seismic data. In the main target strata, the Neogene Shawan Formation can be divided into three fourth-order sequences (SQN₁s₁, SQN₁s₂, and SQN₁s₃), and the sequence SQN₁s₁ is subdivided into three fifth-order sequences (SQN₁s₁¹, SQN₁s₁², and SQN₁s₁³). Based on the established fine-sequence stratigraphic framework, the sedimentary facies types have been identified, they are shallow braided-river deltas, fan deltas, littoral and sublittoral lakes, braided rivers, and terminal fans. Then, stratal slices have been used to clearly depict the boundaries of sedimentary facies. Accurate results have been obtained that characterize braided river channels, terminal fans, littoral and sublittoral lake beaches, and subaqueous distributary channels in the braided-river delta front. Additionally, this seismic sedimentology study reflects variations in source area and evolution history.     

Preliminary research on CBM enrichment models of low-rank coal and its geological controls: A case study in the middle of the southern Junggar Basin,NW China

To date, prospecting work on low-rank coalbed methane (CBM) resources in the middle of the southern Junggar Basin is still in the primary stage, and only a few CBM exploration wells or pilot wells have been deployed in local regions. Systemic understanding of CBM reservoir-forming conditions and geological controlling factors is lacking in the study area, resulting in the mismatch between CBM well deployment and actual geological conditions, as well as poor exploration efficiency. In this paper, the geological controlling effects of the structure, sedimentation, and hydrogeology on CBM enrichment are systematically discussed for the first time, based on the early CBM exploration achievements. The results show that the Xishanyao coal and the Badaowan coal are developed in the upper and lower part of the neutral surface of a fold, respectively. The reservoir-forming conditions of the Badaowan coal are not discussed in this paper due to its poor development. The Xishanyao coal that developed in the axial part of the syncline is most beneficial to CBM enrichment with concentrated extrusion stress and great methane adsorption capacity, while the axial part of the anticline is not favorable for CBM preservation with large tensional stress. The gas content of the Xishanyao thick seams developed in the syncline is higher (average of 4.63–6.34 m³/t) than that in the monocline (average of 2.84–4.56 m³/t). Reverse faulting is more beneficial to CBM enrichment than normal faulting, due to the better sealing capability. The gas content of the Xishanyao coal is obviously influenced by the coal thickness and its roof lithology. The hydrodynamic conditions and total dissolved solids (TDS) values of coalbed water range greatly on regional scale, which leads to a deeper methane weathering zone in the middle-west areas (>1119.62 m) than the eastern Liu-huanggou areas (<501.71 m) and have an important influence on exploration target optimization of CBM exploration wells. Combined with the geological characteristics of the structure, sedimentation and hydrogeology, three CBM enrichment models are proposed in this paper (i.e., broad fold model, northward monocline model and overlying composite model). The reservoir-forming processes and development positions of these CBM enrichment models are discussed systematically to provide a scientific basis for selecting CBM exploration target zones.     

Numerical study of multi-period palaeotectonic stress fields in Lower Cambrian shale reservoirs and the prediction of fractures distribution: A case study of the Niutitang Formation in Feng'gang No. 3 block,South China

Fractures not only control the distribution of oil and gas reservoirs, but also are key points in the research of oil and gas reservoir development programmes. The tectonic fractures in the Lower Cambrian shale reservoirs in the Feng'gang No. 3 block are effective reservoir spaces for hydrocarbon accumulation, and these fractures are controlled by palaeotectonic stress fields. Therefore, quantitatively predicting the development and distribution of tectonic fractures in the Lower Cambrian shale reservoir is important for the exploration and exploitation of shale gas in the Feng'gang No. 3 block. In the present study, a reasonable geological, mechanical and mathematical model of the study area was established based on the faults systems interpreted from seismic data, fracture characteristics from drilling data, uniaxial and triaxial compression tests and experiments on the acoustic emissions (AE) of rocks. Then, a three-dimensional (3-D) finite element method is applied to simulate the palaeotectonic stress field with the superposition of the Yanshan and Himalayan movements and used to predict the fracture distribution. The simulation results indicate that the maximum principal stress value within the study area ranged from 269.97 MPa to 281.18 MPa, the minimum principal stress ranged from 58.29 MPa to 79.64 MPa, and the shear stress value ranged from 91.05 MPa to 106.21 MPa. The palaeotectonic stress field is controlled by the fault zone locations. The fracture development zones are mainly controlled by the tectonic stress fields and are located around the faults, at the end of the fault zones, at the inflection point and at the intersection of the fault zones.     

Distribution characteristics of delta reservoirs reshaped by bottom currents: A case study from the second member of the Yinggehai Formation in the DF1-1 gas field,Yinggehai Basin,South China Sea

The Dongfang1-1 gas field (DF1-1) in the Yinggehai Basin is currently the largest offshore self-developed gas field in China and is rich in oil and gas resources. The second member of the Pliocene Yinggehai Formation (YGHF) is the main gas-producing formation and is composed of various sedimentary types; however, a clear understanding of the sedimentary types and development patterns is lacking. Here, typical lithofacies, logging facies and seismic facies types and characteristics of the YGHF are identified based on high-precision 3D seismic data combined with drilling, logging, analysis and testing data. Based on 3D seismic interpretation and attribute analysis, the origin of high-amplitude reflections is clarified, and the main types and evolution characteristics of sedimentary facies are identified. Taking gas formation upper II (IIU) as an example, the plane distribution of the delta front and bottom current channel is determined; finally, a comprehensive sedimentary model of the YGHF second member is established. This second member is a shallowly buried “bright spot” gas reservoir with weak compaction. The velocity of sandstone is slightly lower than that of mudstone, and the reflection has medium amplitude when there is no gas. The velocity of sandstone decreases considerably after gas accumulation, resulting in an increase in the wave impedance difference and high-amplitude (bright spot) reflection between sandstone and mudstone; the range of high amplitudes is consistent with that of gas-bearing traps. The distribution of gas reservoirs is obviously controlled by dome-shaped diapir structural traps, and diapir faults are channels through which natural gas from underlying Miocene source rocks can enter traps. The study area is a delta front deposit developed on a shallow sea shelf. The lithologies of the reservoir are mainly composed of very fine sand and coarse silt, and a variety of sedimentary structural types reflect a shallow sea delta environment; upward thickening funnel type, strong toothed bell type and toothed funnel type logging facies are developed. In total, 4 stages of delta front sand bodies (corresponding to progradational reflection seismic facies) derived from the Red River and Blue River in Vietnam have developed in the second member of the YGHF; these sand bodies are dated to 1.5 Ma and correspond to four gas formations. During sedimentation, many bottom current channels (corresponding to channel fill seismic facies) formed, which interacted with the superposed progradational reflections. When the provenance supply was strong in the northwest, the area was dominated by a large set of delta front deposits. In the period of relative sea level rise, surface bottom currents parallel to the coastline were dominant, and undercutting erosion was obvious, forming multistage superimposed erosion troughs. Three large bottom current channels that developed in the late sedimentary period of gas formation IIU are the most typical.     

深水复合水道体系沉积特征及时空演化规律——以东非鲁武马盆地中中新统为例

鲁武马盆地中中新统复合水道体系表现出复杂的充填特征及时空演化规律。以地震资料为基础,借助三维可视化、沿层相干切片、层间振幅属性提取等多种地震解释技术,总结地震分辨率下复合水道体系内各个级别水道的平面分布规律,建立纵、横向演化模式。鲁武马盆地中中新统复合水道体系可以细分成4个可识别的级别:复合水道体系,水道复合体,复合水道及水道。沿水流方向,复合水道体系由强限制型向局部限制型演化,在较远端分化为三期独立水道复合体;水道复合体表现出4种沉积方式,2种方式与复合水道体系一致且同步发育,另外2种为较弱限制型和非限制型。复合水道体系内深水沉积主要受海平面变化、地形坡度以及底流作用的影响,沉积规模、搬运距离、沉积位置、延伸方向及外部形态随时空发生改变。复合水道体系呈现出复杂的多级别充填特征,垂向叠置样式随沉积位置的差异而不同,早期水道复合体末期的分布影响后期水道复合体发育的位置。     

Geochemistry,origin, and accumulation of petroleum in the Eocene Wenchang Formation reservoirs in Pearl River Mouth Basin,South China Sea: A case study of HZ25-7 oil field

The Pearl River Mouth Basin in the South China Sea has accumulated >2 km of Eocene sediments in its deep basin, and has become the exploration focus due to the recent discoveries of the HZ25-7 oil field in the Eocene Wenchang (E₂w) Formation. In this study, the geochemical characteristics of potential source rocks and petroleum in the HZ25-7 oil field are investigated and the possible origins and accumulation models developed. The analytical results reveal two sets of potential source rocks, E₂w and Enping (E₂e) formations developed in the study area. The semi-deep-to-deep lacustrine E₂w source rocks are characterized by relatively low C₂₉ steranes, low C₁₉/C₂₃ tricyclic terpane (<0.6), low C₂₄ tetracyclic terpane/C₃₀ hopane (<0.1), low trans-trans-trans-bicadinane (T)/C₃₀ hopane (most <2.0), and high C₃₀ 4-methyl sterane/ΣC₂₉ sterane (>0.2) ratios. In contrast, the shallow lacustrine and deltaic swamp-plain E₂e source rocks are characterized by relatively high C₂₉ steranes, high C₁₉/C₂₃ tricyclic terpane (>0.6), high C₂₄ tetracyclic terpane/C₃₀ hopane (>0.1), variable yet overall high T/C₃₀ hopane, and low C₃₀ 4-methyl sterane/ΣC₂₉ sterane (<0.2) ratios. The relatively low C₁₉/C₂₃ tricyclic terpane ratios (mean value: 0.39), low C₂₄ tetracyclic terpane/C₃₀ hopane ratios (mean value: 0.07), high C₃₀ 4-methyl sterane/ΣC₂₉ sterane ratios (mean value: 1.14), and relatively high C₂₇ regular sterane content of petroleum in the HZ25-7 oil field indicate that the petroleum most likely originated from the E₂w Formation mudstone in the Huizhou Depression. One stage of continuous charging is identified in the HZ25-7 oil field; oil injection is from 16 Ma to present and peak filling occurs after 12 Ma. Thin sandstone beds with relatively good connectivity and physical properties (porosity and permeability) in the E₂w Formation are favorable conduits for the lateral migration of petroleum. This petroleum accumulation pattern implies that the E₂w Formation on the western and southern margins of the Huizhou Depression are favorable for petroleum accumulation because they are located in a migration pathway. Thus exploration should focus in these areas in the future.