Features and genesis of Paleogene high-quality reservoirs in lacustrine mixed siliciclastic–carbonate sediments,central Bohai Sea,China

The characteristics and formation mechanisms of the mixed siliciclastic–carbonate reservoirs of the Paleogene Shahejie Formation in the central Bohai Sea were examined based on polarized light microscopy and scanning electron microscopy observations, X-ray diffractometry, carbon and oxygen stable isotope geochemistry,and integrated fluid inclusion analysis. High-quality reservoirs are mainly distributed in Type Ⅰ and Type Ⅱ mixed siliciclastic–carbonate sediments, and the dominant pore types include residual primary intergranular pores and intrafossil pores, feldspar dissolution pores mainly developed in Type Ⅱ sediments. Type Ⅰ mixed sediments are characterized by precipitation of early pore-lining dolomite, relatively weak mechanical compaction during deep burial, and the occurrence of abundant oil inclusions in high-quality reservoirs. Microfacies played a critical role in the formation of the mixed reservoirs, and high-quality reservoirs are commonly found in high-energy environments, such as fan delta underwater distributary channels,mouth bars, and submarine uplift beach bars. Abundant intrafossil pores were formed by bioclastic decay, and secondary pores due to feldspar dissolution further enhance reservoir porosity. Mechanical compaction was inhibited by the precipitation of pore-lining dolomite formed duringearly stage, and oil emplacement has further led to the preservation of good reservoir quality.     

Hydrocarbon Accumulation in Lacustrine Turbidite in the Rift Basin, Bohai Bay

1. Introduction Turbidite accumulation, which occurs in more than 80 sedimentary basins worldwide (Weimer and Link, 1991), especially in deepwater basins, has come to assume great economic significance in hydrocarbon exploration and become an exploration hotspot to improve petroleum reserves over the last four decades. It is estimated that more than 30 percent of the total reserves discovered worldwide in recent years depend on improvement of deepwater turbidite exploration. These types of di…     

Positive carbon isotope excursions: global correlation and genesis in the Middle–Upper Ordovician in the northern Tarim Basin, Northwest China

Stable carbon isotope ratio(δ~(13) C_(carb)) analysis has been widely applied to the study of the inter-continental or global marine carbonate correlation.Large-scale Cambrian-Ordovician carbonate platforms were developed in the Tarim Basin.But research on fluctuation characteristics and global correlation of(δ~(13) C_(carb)) is still weak.Based on conodont biostratigraphy and whole-rock 8(δ~(13) C_(carb)) data in the Tahe oil-gas field of the northern Tarim Basin,the global correlation and genesis of positive carbon isotope excursions in the Darriwilian—Early Katian was examined.Three positive excursions were identified in the Tahe oil-gas field including the middle Darriwilian carbon isotope excursion(MDICE),the Guttenberg carbon isotope excursion(GICE).and a positive excursion within the Pygodus anserinus conodont zone which is named the Early Sandbian carbon isotope excursion(ESICE) in this paper.Furthermore,these positive excursions had no direct relation with sea level lluctuations.MDICE and GICE could be globally correlated.The Middle-Upper Ordovician Saergan Formation source rocks of the Kalpin outcrops were in accordance with the geological time of MDICE and ESICE.GICE had close relationship with the source rock of the Lianglitag Formation in the basin.Massive organic carbon burial was an important factor controlling the genesis of these positive excursions.     

Geological characteristics and accumulation mechanisms of the “continuous” tight gas reservoirs of the Xu2 Member in the middlesouth transition region, Sichuan Basin, China

“Continuous” tight gas reservoirs are those reservoirs which develop in widespread tight sandstones with a continuous distribution of natural gas. In this paper, we summarize the geological features of the source rocks and “continuous” tight gas reservoirs in the Xujiahe Formation of the middlesouth transition region, Sichuan Basin. The source rocks of the Xu1 Member and reservoir rocks of the Xu2 Member are thick (Xu1 Member: 40 m, Xu2 Member: 120 m) and are distributed continuously in this study area. The results of drilled wells show that the widespread sandstone reservoirs of the Xu2 Member are charged with natural gas. Therefore, the natural gas reservoirs of the Xu2 Member in the middle-south transition region are “continuous” tight gas reservoirs. The accumulation of “continuous” tight gas reservoirs is controlled by an adequate driving force of the pressure differences between source rocks and reservoirs, which is demonstrated by a “one-dimensional” physical simulation experiment. In this simulation, the natural gas of “continuous” tight gas reservoirs moves forward with no preferential petroleum migration pathways (PPMP), and the natural gas saturation of “continuous” tight gas reservoirs is higher than that of conventional reservoirs.     

Oil–source rock correlation and quantitative assessment of Ordovician mixed oils in the Tazhong Uplift, Tarim Basin

The origin of the marine oils in the Tarim Basin has long been a disputed topic. A total of 58 DST (drill stem test) crude oil and 8 rock samples were investigated using a comprehensive geochemical method to characterize and identify the origin of the Ordovician oils in the Tazhong Uplift, Tarim Basin, northwest China. Detailed oil–oil and oil–source rock correlations show that the majority of the oils have typical biomarker characteristics of the Middle-Upper Ordovician (O2+3) source rock and the related c...     

Characteristics of Oil and Gas Accumulation in Yong’an-Meitai Area of the Fushan Depression, Beibuwan Basin, South China Sea

The Yong’an-Meitai area is the focus of the present exploration in the Fushan Depression, Beibuwan Basin, South China Sea. All oils from this area are geochemically characterized by higher Pr/Ph ratio, higher proportion of heavy molecular weight hydrocarbons, and higher proportion of C29 regular steranes, which indicate that the organic matter of source rocks might have been deposited in an oxidizing palaeoenvironment and be dominated by higher plant organic matter input. The oil from E3w2 (the second member of Weizhou Fm. of the Oligocene) has a much higher density, relatively higher Pr/nC17 and Ph/nC18 ratios, and a “UCM—unresolved complex mixture” on gas chromatograms, which indicate that it has been slightly biodegraded. CPI and other terpane and sterane isomer ratios suggest they are all mature oils. The timing of oil charging in E3w2 and E2l1 (the first member of the Liushagang Fm. of the Eocene) determined by the homogenization temperatures of fluid inclusions and thermal evolution history are from 9-3 Ma and 8-3 Ma, respectively. Thus, the interpretation of E3w2 as a secondary reservoir is unlikely. The timing of oil charging is later than that of hydrocarbon generating and expulsion of Liushagang Fm. source rocks and trap formation, which is favorable for oil accumulation in this area. All molecular parameters that are used for tracing oil filling direction decrease with shallower burial depth, which suggests vertical oil migration. The widely occurring faults that penetrate through the source rocks of the Liushagang Fm. may serve as a fine oil charging conduit.     

Depositional Patterns and Oil／Gas Accumulation Features of Sha-3 Member Turbidites in Dongying Depression, Bohai Bay Basin

近年来东营凹陷的油气勘探结果表明该凹陷仍有巨大的勘探潜力,未发现的油气主要蕴藏于包括沙三段浊积岩性圈闭在内的隐蔽圈闭内。为了指导浊积岩性圈闭的油气勘探,重点探讨了沙三段浊流砂体的沉积模式和成藏特征。文章的独特之处在于作者建立了两个类比关系:沙三段沉积时的沉积格架与东非裂谷半地堑的现代沉积相似,沙三段3类不同的湖相浊积岩(陡坡带浊积砂砾岩扇裙、缓坡带湖底扇和凹陷带三角洲前缘浊积砂体)的沉积模式可以与表征海相沉积岩的沉积模式相类比。陡坡带浊积砂砾岩和缓坡带湖底扇浊积岩能否成藏的关键要素是上倾方向是否存在封堵条件,而控制三角洲前缘浊积砂体能否成藏的要素不是很清楚,可能有多个因素。     

Characteristics of Guang’an Xu6 Tight Gas Reservoir in the Middle of Sichuan Basin,China

Tight gas reservoirs have been researched nearly 50 years and become more and more important in the future with the increasing consumption of nature gas. In this study, the characteristics of Guang’an tight gas reservoir, which lies in the middle of Sichuan Basin, China, was analyzed. This reservoir is a typical tight gas reservoir. First, the porosity and permeability of it are low. Second, it has high water saturation and diffused boundaries. Third, it does not have an obvious seal. Fourth, it does not have a clear gas-water contact. Fifth, it has geologic “sweet spots,” close to source rock. Sixth, it has normal reservoir pressure. According to the actual conditions of Guang’an tight gas reservoir, a new reservoir accumulation model is established, which integrates the knowledge of conventional and unconventional reservoir, and which can explain the special phenomenon that tight gas reservoir can produce water and have normal pressure.     

Sedimentary environment controls on the accumulation of organic matter in the Upper Ordovician Wufeng–Lower Silurian Longmaxi mudstones in the Southeastern Sichuan Basin of China

The Upper Ordovician–Lower Silurian mudstones(including the Wufeng, Guanyinqiao and Longmaxi Formations) in the Sichuan Basin are some of the most important shale gas plays in China. In order to enhance our understanding of the process of formation of organic carbon up to 10%, optical, microscopy and geochemical methods have been used to investigate the petrographic and geochemical characteristics of the formation. Firstly, three mudstone lithofacies were identified based on a wide variety of mudstone laminations. These are:(a) indistinctly laminated mudstone;(b) parallellaminated mudstone; and(c) nonparallel-laminated mudstone. Then, combining with the evidence from depocenter migration, Th/U ratios and total organic carbon, the abundant organo-minerallic fabrics suggest that organic carbon was preferentially deposited and preserved in anoxic, low energy and stagnant water conditions during deposition of the Wufeng and Longmaxi Formations. On the contrary, the Guanyinqiao Formation with poor organic carbon was deposited in oxic and high-energy water conditions.     

Existence and implications of hop-17（21）-enes in the lower Cretaceous of the Saihantala Sag, Erlian Basin, China

C31- to C35-hop-17(21)-enes are identified by gas chromatography-mass spectrometry (GC-MS) analysis to exist as double isomers in most samples of the Aershan Formation and members 1 and 2 of the Tenggeer Formation from well SH3. Comprehensive organic geochemistry and organic petrology study indicates that algae and bacteria are the main biological source of lower Cretaceous sediments in the Saihantala Sag, and this is in accordance with the existence of hop-17(21)-enes. The similar distributions of hop-17(21)-enes and hopanes of these samples indicate that hop-17(21)-enes were transformed into hopanes through hydrogenation during diagenesis processes. The existence of hop-17(21)-enes means that not only the formation of organic matter is related to an anoxic environment and a biological source of algae and bacteria, but also hop-17(21)-enes are direct indicators of hydrocarbon rock at an immature to low-maturity stage. High hydrocarbon conversion ratio, algae and bacteria source and a high abundance of organic matter suggest that the Saihantala Sag has the potential to generate immature to low-maturity oil, which may be of great significance for oil exploration in the Erlian Basin.     

Sedimentary Characteristics and Evolution of Asri Basin,Indonesia,in Early Tertiary

1.Introduction The Asri basin is oval in shape,covering about2500km2from north to south.The maximum thickness at the depocenter is around18,000feet(5,500m).It is adjacent to the Hera basin in the west and to the Sunda basin in the southwest(Fig.1).Now it is under the Java sea.The basement consists of granites,granodiorites,marbles,schists and quartzites(Tonkin and Himawan,1999).In the early Tertiary,the eastern side of the basin was a steep slope controlled by syn-rifting,and the western s…     

Crude oil and condensate in the Ordovician–Silurian formations of the Eastern European: The Western Black Sea Basin

The Lower Paleozoic sediments of Eastern Europe extend from Poland to the Black Sea Coasts. The Lower Ordovician Bakacak Formation and Middle Ordovician–Silurian Karadere, Ketencikdere, and F?nd?kl? formations of the Western Black Sea Basin contain mudstones and black shales with sandstone and limestone intervals. The mudstones of the Lower Ordovician do not display any potential. But some intervals of the Middle Ordovician–Silurian black shales have potential for unconventional exploration because total organic carbon (TOC) of organic matter contents ranges from 0.11% to 1.94%. Hydrogen index (HI) values of the Karadere, Ketencikdere, and F?nd?kl? formations are ranging 0–139 mg oil/g TOC. Vitrinite reflection equivalent (Roe) estimation from HI shows that the shales locate within the oil, wet gas-condensate, and dry gas zone. In-situ oil and condensate volumes of the black shales have been calculated between 0 and 2812 mg HC/g TOC. These volumes indicate presence of potentially producible hydrocarbon in the some intervals of the Middle Ordovician–Silurian shales. Mineralogical properties of the Karadere, Ketencikdere, and F?nd?kl? formations are suitable for hydraulic fracturing because the shales are dominated by quartz/carbonate. Quartz ratio ranges from 12% to 91% with an average 53%. Total calcite/dolomite content is an average of 13/0.5%. Total clay mineral chance between 0% and 39% with an average of 21%.     

Seismic damage zone and width–throw scaling along the strike-slip faults in the Ordovician carbonates in the Tarim Basin

Understanding the scaling relation of damage zone width with displacement of faults is important for predicting subsurface faulting mechanisms and fluid flow processes. The understanding of this scaling relationship is influenced by the accuracy of the methods and types of data utilized to investigate faults. In this study, seismic reflection data are used to investigate the throw and damage zone width of five strike-slip faults a ecting Ordovician carbonates of the Tarim intracraton basin,NW China. The results indicate that fault slips with a throw less than 200 m had formed wide damage zones up to 3000 m in width. Also, damage zone width is found to have both a positive correlation and a power-law relation with throw of two orders of magnitude, with a ratio of these values varying in a range of 2–15. The relationship between throw and damage zone width is not a simple power-law and changes its slope from small to larger size faults. The results indicate that throw scales well with damage zone width for the studied faults, and hence these can be used to predict fault geometries in the Tarim Basin. The study of the wide carbonate damage zones presented here provides new insights into scaling of large-size faults, which involve multiple faulting stages.     

Selective dissolution of eodiagenesis cements and its impact on the quality evolution of reservoirs in the Xing’anling Group, Suderte Oil Field, Hailar Basin, China

Reservoirs in the Xing’anling Group in the Suderte Oil Field, Hailar Basin exhibit ultra-low to low permeability and high tuffaceous material content. This study comprehensively analyzed diagenesis and quality evolution of these low-permeability reservoirs using thin sections, SEM samples, rock physical properties, pore water data, as well as geochemical numerical simulations. Calcite and analcite are the two main types of cements precipitated in the eodiagenetic stage at shallow burial depths in the reservoirs. These two cements occupied significant primary intergranular pores and effectively retarded deep burial compaction. Petrography textures suggest selective dissolution of massive analcite and little dissolution of calcite in the mesodiagenetic stage. Chemical calculations utilizing the Geochemist’s Workbench 9.0 indicated that the equilibrium constant of the calcite leaching reaction is significantly smaller than that of the analcite leaching reaction, resulting in extensive dissolution of analcite rather than calcite in the geochemical system with both minerals present. Numerical simulations with constraints of kinetics and pore water chemistry demonstrated that the pore water in the Xing’anling group is saturated with respect to calcite, but undersaturated with analcite, leading to dissolution of large amounts of analcite and no dissolution of calcite. Significant secondary intergranular pores have formed in analcite-cemented reservoirs from selective dissolution of analcite in the mesodiagenetic stage; the analcite dissolution formed preferential flow paths in the reservoirs, which promoted feldspar dissolution; and dissolution of such minerals led to the present reservoirs with medium porosity and low permeability. Calcite-cemented tight reservoirs have not experienced extensive dissolution of cements, so they exhibit ultra-low porosity and permeability.     

Paleoporosity and critical porosity in the accumulation period and their impacts on hydrocarbon accumulation—A case study of the middle Es3 member of the Paleogene formation in the Niuzhuang Sag,Dongying Depression,Southeastern Bohai Bay Basin,East Chi

Similar reservoir sandbodies and fault conduit systems in the sandstone reservoirs in the middle Es3 member of the Niuzhuang Sag have been problematic for a long time. The following problems remain unsolved： 1） The distribution of sandstone porosity is inconsistent with the hydrocarbon accumulation. The oil sandstones have low porosity instead of high porosity. 2） Sandstones, which have the same properties, have different levels of oiliness, and the sandstones with almost the same properties show different degrees of oil-bearing capacity. This study analyzes the condition of reservoirs in the research area during the accumulation period in terms of paleoporosity estimation and discusses the critical porosity of the sandstone reservoirs during the same period. The following conclusions can be drawn from the results. 1） Although reservoir properties are low at present and some reservoirs have become tight, the paleoporosity ranging from 18% to 25% is greater than the critical porosity of 13.9%. As the： loss of porosity is different in terms of burial history, the present porosity cannot reflect porosity during the accumulation period. Similar/y, high porosity during the accumulation period does not indicate that tbe present porosity is high. 2） The present reservoir location is consistent with the distribution of high paleoporosity during the accumulation period. This result indicates that high porosity belts are prone to hydrocarbon accumulation because of the dominant migration pathways generated as a result of property discrepancies under similar fault conduit conditions. Consequently, the hydrocarbon mainly accumulates in high porosity belts. Paleoporosity during the accumulation period is found to be a vital controlling factor. Therefore, high paleoporosity sandstones in the middle Es3 member of the Niuzhuang Sag have great potential for future exploration.     

Characteristics of the lower part of the Xiaganchaigou Formation in the Mabei-Ⅶ structure in the northern margin, Qaidam Basin

基于岩心、测井资料及普通薄片、铸体薄片、扫描电镜、X衍射、物性和压汞等化验分析资料,研究了柴达木盆地马北8号构造下干柴沟组下段砂岩储层特征.结果表明:该套储层的储集空间以原生粒间孔为主,夹少量次生粒间孔,储层物性好,排驱压力和饱和度中值压力较低,孔喉半径较大且分选性中等,歪度粗,孔隙结构参数总体较好.储层质量受沉积相和成岩作用的综合影响,其中,辫状河河床及滩坝沉积区为最有利储层发育区,加之压实作用较弱、成岩早期的中等胶结作用和比较普遍的溶蚀作用有利于原生孔隙和喉道的保存及部分次生孔隙和喉道的发育.利用储层物性及压汞等参数,将马北8号构造下干柴沟组下段储层划分为Ⅰ、Ⅱ、Ⅲ类储层,其中Ⅰ、Ⅱ类储层在目前的技术条件下是增储上产的首选储层.     

Reservoir characterization of the Odovician oil and gas pools in the Tahe Oilfield, Tarim Basin, Northwest China

About 88. 1% of the proven reserves in the Tahe Oilfield in the Tarim Basin of Northwest China are trapped in Ordovician carbonate reservoirs. These reservoirs are formed by unconnected and interconnected networks of karstic porosity forming a heterogeneous and complex reservoir system. Oil, water and gas characteristics vary significantly in different portions of the Ordovician reservoir. There is no uniform oil/water contact in the field, adding to its complexity.An acid fracture treatment is beneficial in 76% of the wells, stimulating nonproductive wells and enhancing production in other wells by fracturing into unconnected reservoirs and enhancing flow pathways with acid. Acid fracture treatments should be a standard procedure for developing this and similar oil fields.     

Significance of Coal Seam in Recognition of Sequence Boundaries and Systems Tract of the Jurassic in East Fukang Slope, Junggar Basin, China

1. Introduction The East Fukang Slope is located in the east of the Changji Sag and on the junction of the middle-south of the Zhangbei Fault-fold Zone and the Baijiahai Salient in the northeast of the Junggar Basin, Northwest China, with an area of about 5,000 square kilometres (Fig. 1). When the Jurassic was deposited, a huge slope with a north-south dip was formed, which consisted of the Wucaiwan Sag, front of the Kelameili Mountain piedmont, the Baijiahai Salient and the East Fuka…     

Application of 17α(H)-diahopanes in oil-source correlation of Triassic Yanchang formation in Zhoujiawan area,Ordos Basin,China

17α(H)-diahopane are extensively distributed in Yanchang formation source rocks of Ordos Basin, but the content varies greatly. The content of 17α(H)-diahopane is pretty low in Chang 7 oil shale, whereas it is very high in Chang 9 mudstones. Low diahopane abundance of Chang 8₁ reservoir extracts and similar crossplots of Ts/C29Ts and C₃₀ diahopane/C₃₀ hopane show that Chang 8₁ oils come from Chang 7 oil shale. However, Chang 8₂ and Chang 9 reservoir extracts are similar to Chang 9 source rocks in most of geochemical parameters, which indicates that the oil is derived from Chang 9 source rocks.