Geochemical overview and undiscovered gas resources generated from Hamitabat petroleum system in the Thrace Basin,Turkey

Since the first drill in 1957, three oil, 19 gas and condensate fields have been discovered in the Thrace Basin. However, any petroleum system with its essential elements and processes has not been assigned yet. This study consists of two parts, (1) geochemical overview of the previous work in order to get a necessary help to construct a petroleum system and (2) calculation of quantitative undiscovered hydrocarbon resources generated from this system. An extensive overview study showed that the primary reservoir and source rocks in the Thrace Basin are the Middle Eocene Hamitabat sandstones and shales, respectively, hence it appears that the most effective petroleum system of the Thrace Basin becomes the Hamitabat (!) petroleum system. Currently, 18.5 billion m³ of in-place gas, 2.0 million m³ (12.7 million bbl) in-place waxy oil as well as minor amount of associated condensate were discovered from this system. This study showed that the regional distribution of the oil and gas fields almost overlapped with the previously constructed pod of active Hamitabat shales implying that short and up-dip vertical migration pathway of hydrocarbons from the source to trapping side was available. Thermal model demonstrated that hydrocarbon generation from the Hamitabat shales commenced in the Early Miocene. The amount of quantitative gas generation based on the mean-original TOC = 0.94 wt%, mean-original HI = 217 HC/g TOC and the volume of the pod of active source rock = 49 km³ is approximately 110 billion m³ of gaseous hydrocarbons that results in a high generation–accumulation efficiency of 17% when 18.5 billion m³ of already discovered hydrocarbons are considered.     

The thermal structure of the Dvurechenskii mud volcano and its implications for gas hydrate stability and eruption dynamics

The sediment temperature distribution at mud volcanoes provides insights into their activity and into the occurrence of gas hydrates. If ambient pressure and temperature conditions are close to the limits of the gas hydrate stability field, the sediment temperature distribution not only limits the occurrence of gas hydrates, but is itself influenced by heat production and consumption related to the formation and dissociation of gas hydrates. Located in the Sorokin Trough in the northern Black Sea, the Dvurechenskii mud volcano (DMV) was in the focus of detailed investigations during the M72/2 and M73/3a cruises of the German R/V Meteor and the ROV Quest 4000 m in February and March 2007. A large number of in-situ sediment temperature measurements were conducted from the ROV and with a sensor-equipped gravity corer. Gas hydrates were sampled in pressurized cores using a dynamic autoclave piston corer (DAPC). The thermal structure of the DMV suggests a regime of fluid flow at rates decreasing from the summit towards the edges of the mud volcano, accompanied by intermittent mud expulsion at the summit. Modeled gas hydrate dissociation temperatures reveal that the gas hydrates at the DMV are very close to the stability limits. Changes in heat flow due to variable seepage rates probably do not result in changes in sediment temperature but are compensated by gas hydrate dissociation and formation.     

Winkler's method overestimates dissolved oxygen in seawater: Iodate interference and its oceanographic implications

Dissolved oxygen in seawater has been determined by using the Winkler's reaction scheme for decades. An interference in this reaction scheme that has been heretofore overlooked is the presence of naturally occurring iodate in seawater. Each mole of iodate can result in an apparent presence of 1.5 mol of dissolved oxygen. At the concentrations of iodate in the surface and deep open ocean, it can lead to an overestimation of 0.52 ± 0.15 and 0.63 ± 0.05 μmol kg^− 1 of oxygen in these waters respectively. In coastal and inshore waters, the effect is less predictable as the concentration of iodate is more variable. The solubility of oxygen in seawater was likely overestimated in data sources that were based on the Winkler's reaction scheme for the determination of oxygen. The solubility equation of García and Gordon [Garcia H.E., Gordon, L.I., 1992. Oxygen solubility in seawater: Better fitting equations. Limnol. Oceanogr. 37, 1307–1312] derived from the results of Benson and Krause [Benson, F.B., Krause, D. Jr., 1984. The concentration and isotopic fractionation of oxygen dissolved in freshwater and seawater in equilibrium with the atmosphere. Limnol. Oceanogr. 29, 620–632] is free from this source of error and is recommended for general use. By neglecting the presence of iodate, the average global super-saturation of oxygen in the surface oceans and the corresponding efflux of oxygen to the atmosphere both have been overestimated by about 8%. Regionally, in areas where the degree of super-saturation or under-saturation of oxygen in the surface water is small, such as in the tropical oceans, the net air–sea exchange flux can be grossly under- or overestimated. Even the estimated direction of the exchange can be reversed. Furthermore, the presence of iodate can lead to an overestimation of the saturation anomaly of oxygen in the upper ocean attributed to biological production by 0.23 ± 0.07%. AOU may have been underestimated by 0.52 ± 0.15 and 0.63 ± 0.05 μmol kg^− 1 in the surface mixed layer and deep water, while preformed phosphate and preformed nitrate may have been overestimated by 0.004 ± 0.001 and 0.06 ± 0.02 μmol kg^− 1 in the surface mixed layer, and 0.005 ± 0.0004 and 0.073 ± 0.006 μmol kg^− 1 in the deep water. These are small but not negligible corrections, especially in areas where the values of these parameters are small. At the increasing level of sophistication in the interpretation of oxygen data, this source of error should now be taken into account. Nevertheless, in order to avoid confusion, an internationally accepted standard needs to be adopted before these corrections can be applied.     

Seabed morphology and gas venting features in the continental slope region of Krishna–Godavari basin,Bay of Bengal: Implications in gas-hydrate exploration

Increased oil and gas exploration activity has led to a detailed investigation of the continental shelf and adjacent slope regions of Mahanadi, Krishna–Godavari (KG) and Cauvery basins, which are promising petroliferous basins along the eastern continental margin of India. In this paper, we analyze the high resolution sparker, subbottom profiler and multibeam data in KG offshore basin to understand the shallow structures and shallow deposits for gas hydrate exploration. We identified and mapped prominent positive topographic features in the bathymetry data. These mounds show fluid/gas migration features such as acoustic voids, acoustic chimneys, and acoustic turbid layers. It is interesting to note that drilling/coring onboard JOIDES in the vicinity of the mounds show the presence of thick accumulation of subsurface gas hydrate. Further, geological and geochemical study of long sediment cores collected onboard Marion Dufresne in the vicinity of the mounds and sedimentary ridges shows the imprints of paleo-expulsion of methane and sulfidic fluid from the seafloor.     

Marine hydrocarbon source rocks of the Upper Permian Longtan Formation and their contribution to gas accumulation in the northeastern Sichuan Basin,southwest China

Identification of the main hydrocarbon source rocks of the large Puguang gas field (northeastern Sichuan Basin, southwest China) has been the subject of much discussion in recent years. A key aspect has been the lack of a comprehensive understanding of the development of hydrocarbon source rocks of the Upper Permian Longtan Formation, which had been thought to contain mainly coal seams and thick carbonate layers. In this paper, based on geological data from more than ten wells and outcrops and their related mineralogy and geochemistry, we investigated the depositional environment and main factors controlling organic matter enrichment in the Longtan Formation. We propose a model which combines information on the geological environment and biological changes over time. In the model, organic matter from prolific phytoplankton blooms was deposited in quiescent platform interior sags with rising sea-levels. During the Longtan period, the area from Bazhong to Dazhou was a platform interior sag with relatively deep water and a closed environment, which was controlled by multiple factors including syngenetic fault settling, isolation of submarine uplifts and rising sea-levels leading to water column stratification. Although the bottom water was anoxic, the phytoplankton were able to bloom in the well-lit upper euphotic zone thus giving rise to a set of sapropelic black shales and marlstones containing mostly algal organic matter with minor terrestrial contributions. As a consequence, these rocks have a high hydrocarbon generation potentials and can be classified as high-quality source rocks. The area from Bazhong to Dazhou is a center of hydrocarbon generation, being the main source of reservoired paleo-oils and presently discovered as pyrobitumen in the Puguang gas field. The identification of these source rocks is very important to guide future petroleum exploration in the northeastern Sichuan Basin.     

Reservoir origin and characterization of gas pools in intrusive rocks of the Yingcheng Formation,Songliao Basin,NE China

In 2013, the first discovery of gas pools in well LS 208 in intrusive rocks of the Songliao Basin (SB), NE China was made in the 2nd member of the Yingcheng Formation in the Yingtai rift depression, proving that intrusive rocks of the SB have the potential for gas exploration. However, the mechanisms behind the origin of reservoirs in intrusive rocks need to be identified for effective gas exploration. The gas pool in intrusive rocks can be characterized as a low-abundance, high-temperature, normal-pressure, methane-rich, and lithologic pool based on integrated coring, logging, seismic, and oil test methods. The intrusive rocks show primary and secondary porosities, such as shrinkage fractures (SF), spongy pores (SP), secondary sieve pores (SSP), and tectonic fractures (TF). The reservoir is of the fracture–pore type with low porosity and permeability. A capillary pressure curve for mercury intrusion indicates small pore-throat size, negative skewness, medium–high displacement pressure, and middle–low mercury saturation. The development of fractures was found to be related to the quenching effects of emplacement and tectonic inversion during the middle–late Campanian. SP and SSP formed during two phases. The first phase occurred during emplacement of the intrusive rock in the late Albian, when the intrusions underwent alteration by organic acids. The second phase occurred between the early Cenomanian and middle Campanian, when the intrusions underwent alteration by carbonic acid. The SF formed prior to oil charging, the SSP + SP formed during oil charging, and the TF formed during the middle–late Campanian and promoted the distribution of gas pools throughout the reservoir. The intrusive rocks in the SB and the adjacent basins were emplaced in the mudstone and coal units, and have great potential for gas exploration.     

Pressure significance of gas chimneys

Analysis was carried out of part of the northern North Sea to test what the presence and style of gas chimneys indicate about fluid pressure (P_f) within hydrocarbon reservoirs. Previous results suggest that broad chimneys above a trap and thin chimneys on the flanks indicate the presence of hydrocarbons, whilst thin chimneys in the crest suggest the hydrocarbons have escaped. Each type of gas chimney is usually associated with overpressure within Mesozoic reservoirs, but the water leg is hydrostatically-pressured in most Cenozoic reservoirs. This indicates: (a) gas leaking from a trap does not necessarily cause P_f to become hydrostatic; (b) overpressure may not be necessary for the expulsion of gasses through seal units to create the chimneys; (c) although gas chimneys indicate the existence of an active hydrocarbon system, their presence does not appear to indicate anything significant about present-day P_f.     

A laboratory study on capillary sealing efficiency of Iranian shale and anhydrite caprocks

Caprock has the most important role in the long term safety of formation gas storage. The caprocks trap fluid accumulated beneath, contribute to lateral migration of this fluid and impede its upward migration. The rapid upward passage of invasive plumes due to buoyancy pressure is prevented by capillary pressure within these seal rocks. In the present study, two main seal rocks, from the Zagros basin in the southwest of Iran, a shale core sample of Asmari formation and an anhydrite core sample of Gachsaran formation, were provided. Absolute permeabilities of shale and anhydrite cores, considering the Klinkenberg effect, were measured as 6.09 × 10⁻¹⁸ and 0.89 × 10⁻¹⁸ m², respectively. Capillary sealing efficiency of the cores was investigated using gas breakthrough experiments. To do so, two distinct techniques including step by step and residual capillary pressure approaches were performed, using carbon dioxide, nitrogen and methane gases at temperatures of 70 and 90 °C, under confining pressures in the range 24.13–37.92 MPa. In the first technique, it was found that capillary breakthrough pressure of the cores varies in the range from 2.76 to 34.34 MPa. Moreover, the measurements indicated that after capillary breakthrough, gas effective permeabilities lie in range 1.85 × 10⁻²¹ – 1.66 × 10⁻¹⁹ m². In the second technique, the minimum capillary displacement pressure of shale varied from 0.66 to 1.45 MPa with the maximum effective permeability around 7.76 × 10⁻²¹ – 6.69 × 10⁻²⁰ m². The results indicate that anhydrite caprock of the Gachsaran formation provides proper capillary sealing efficacy, suitable for long term storage of the injected CO₂ plumes, due to its higher capillary breakthrough pressure and lower gas effective permeability.     

海洋细菌脂肪酸的气相色谱分析

以副溶血性弧菌VP-X-3为试验对象,利用气相色谱技术分析了其细胞脂肪酸的组成和含量,以色谱峰的数量、色谱峰高及脂肪酸含量作为评价指标,探讨了不同的皂化和甲酯化条件对细菌细胞脂肪酸测定结果的影响。结果表明:皂化和甲酯化条件都对细胞脂肪酸的组成和含量产生一定的影响。较低温度皂化处理利于增加色谱峰数量和色谱峰高。甲酯化温度过高容易破坏脂肪酸,尤其是长链脂肪酸和不饱和脂肪酸的结构。高温甲酯化条件下,时间越长,甲酯化效果越差。试验得到较为理想的气相色谱前处理条件:2 mol/L NaOH-甲醇溶液2 mL,70℃水浴10 min;10%(V/V)H2SO4-甲醇溶液2 mL,70℃水浴15 min。此条件下的色谱峰的数量多达29种,色谱峰高明显高于其他处理条件下的峰高。     

Large-scale depositional characteristics of the Ulleung Basin and its impact on electrical resistivity and Archie-parameters for gas hydrate saturation estimates

Gas hydrate saturation estimates were obtained from an Archie-analysis of the Logging-While-Drilling (LWD) electrical resistivity logs under consideration of the regional geological framework of sediment deposition in the Ulleung Basin, East Sea, of Korea. Porosity was determined from the LWD bulk density log and core-derived values of grain density. In situ measurements of pore-fluid salinity as well as formation temperature define a background trend for pore-fluid resistivity at each drill site. The LWD data were used to define sets of empirical Archie-constants for different depth-intervals of the logged borehole at all sites drilled during the second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2). A clustering of data with distinctly different trend-lines is evident in the cross-plot of porosity and formation factor for all sites drilled during UBGH2. The reason for the clustering is related to the difference between hemipelagic sediments (mostly covering the top ∼100 mbsf) and mass-transport deposits (MTD) and/or the occurrence of biogenic opal. For sites located in the north-eastern portion of the Ulleung Basin a set of individual Archie-parameters for a shallow depth interval (hemipelagic) and a deeper MTD zone was achieved. The deeper zone shows typically higher resistivities for the same range of porosities seen in the upper zone, reflecting a shift in sediment properties. The presence of large amounts of biogenic opal (up to and often over 50% as defined by XRD data) was especially observed at Sites UBGH2-2_1 and UBGH2-2_2 (as well as UBGH1-9 from a previous drilling expedition in 2007). The boundary between these two zones can also easily be identified in gamma-ray logs, which also show unusually low readings in the opal-rich interval. Only by incorporating different Archie-parameters for the different zones a reasonable estimate of gas hydrate saturation was achieved that also matches results from other techniques such as pore-fluid freshening, velocity-based calculations, and pressure-core degassing experiments. Seismically, individual boundaries between zones were determined using a grid of regional 2D seismic data. Zoning from the Archie-analysis for sites in the south-western portion of the Ulleung Basin was also observed, but at these sites it is linked to individually stacked MTDs only and does not reflect a mineralogical occurrence of biogenic opal or hemipelagic sedimentation. The individual MTD events represent differently compacted material often associated with a strong decrease in porosity (and increase in density), warranting a separate set of empirical Archie-parameters.