Petroleum Accumulation Associated with Volcanic Activity in the Tarim Basin——Taking Tazhong-47 Oilfield as an Example

塔中47油田的发现证实在塔里木盆地围绕火成岩寻找油气田是现实的。火山活动以及由此形成的火成岩体对油气聚集的有利条件主要体现在两个方面:一是沿火成岩与围岩的接触面构成油气运移的通道,火成岩与围岩接触,似一直立的不整合面,与孔渗性好的岩层组合不仅能使油气运移而且遇有圈闭油气还会聚集成藏;二是构成火山岩遮挡的圈闭,或是对原有圈闭进行改造、分割。另外,火成岩也是一类潜在的储集层,在国内外都有不少实例,塔中隆起中-1井在二叠系火成岩中也见到油气显示。与火山活动相关的油气聚集将成为塔里木盆地一个新的勘探领域。     

Practical effect of the “transfer station” model for oil-gas migration in rift basin:A case study on the Tertiary in the Bohai oil province

断层是裂谷盆地重要的油气运移通道.对渤海油区大量成功与失利探井的深入分析表明,单纯断面的运移能力并不强,大断层只有与烃源岩内的“中转站”砂体配合,才能为上部圈闭提供充足的油源;凹陷内小断层根部无“中转站”,只能形成小油藏;直立的走滑断层下部无“中转站”,其上部无油藏.深入研究断层的运移能力,会大幅提高勘探成功率.     

Identification of the lower limit of high-quality source rocks and its relation to hydrocarbon accumulation— Taking the Beier Sag in the Hailaer Basin as an example

The theory of “source rock control” has evolved from source-rock-control hydrocarbon accumulation, to effective source-rock-control hydrocarbon accumulation, and to high-quality sourcerock- control hydrocarbon accumulation. However, there are problems, such as whether high-quality source rocks exist or not? What high-quality source rocks are, and how to identify them, are yet to be agreed upon. Aimed at this issue of concern to explorationists, and taking the Beier Sag in the Hailaer Basin as an example, this paper defines the high-quality source rocks and the lower limit for evaluation of high-quality source rocks, by using the inflection point on the relationship curve of hydrocarbon (oil) expulsion, which is calculated by the material balance principle, versus total organic carbon (TOC). The results show that when TOC is low, all source rocks have limited hydrocarbon expulsion and slow growth rate, thus they cannot be high-quality source rocks. However, when TOC rises to some threshold, hydrocarbon expulsion increases significantly with TOC. This inflection point should be the lower limit of high-quality source rocks: those with TOC greater than the inflection-point value are high-quality source rocks. In addition, the lower limit of high-quality source rocks is also related to the type and maturity of organic matters in the source rocks, as well as the mineral components of the source rocks affecting the residual hydrocarbons. Theoretically, the lower limit of high-quality source rocks depends on geological conditions rather than being a constant value. However, for the sake of simplicity and practicability, in this paper TOC=2.0% is regarded as the lower limit of high-quality source rocks. The examination of such standard in the work area indicates that the high-quality source rocks in members K1n2 and K1n1 of the Nantun formation contribute 76% and 82% to oil generation, and 96% and 91% to oil expulsion , respectively. The distribution of high-quality source rocks is also closely related to the distribution of hydrocarbon reservoirs in the region, demonstrating that high-quality source rocks control hydrocarbon accumulation.     

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.     

Hydrocarbon migration and accumulation along the fault intersection zone——a case study on the reef-t systems of the No.1 slope break zone in the Tazhong area, Tarim Basin

Understanding hydrocarbon migration and accumulation mechanisms is one of the key scientific problems that should be solved for effective hydrocarbon exploration in the superimposed basins developed in northwest China.The northwest striking No.1 slope break zone,which is a representative of superimposed basins in the Tarim Basin,can be divided into five parts due to the intersection of the northeast strike-slip faults.Controlled by the tectonic framework,the types and properties of reservoirs and the hydrocarbon compositions can also be divided into five parts from east to west.Anomalies of all the parameters were found on the fault intersection zone and weakened up-dip along the structural ridge away from it.Thus,it can be inferred that the intersection zone is the hydrocarbon charging position.This new conclusion differs greatly from the traditional viewpoint,which believes that the hydrocarbon migrates and accumulates along the whole plane of the No.1 slope break zone.The viewpoint is further supported by the evidence from the theory of main pathway systems,obvious improvement of the reservoir quality (2-3 orders of magnitude at the intersection zone) and the formation mechanisms of the fault intersection zone.Differential hydrocarbon migration and entrapment exists in and around the strikeslip faults.This is controlled by the internal structure of faults.It is concluded that the more complicated the fault structure is,the more significant the effects will be.If there is a deformation band,it will hinder the cross fault migration due to the common feature of two to four orders of magnitude reduction in permeability.Otherwise,hydrocarbons tend to accumulate in the up-dip structure under the control of buoyancy.Further research on the internal fault structure should be emphasized.     

Hydrocarbon migration and accumulation along the fault intersection zone—a case study on the reef-flat systems of the No.1 slope break zone in the Tazhong area, Tarim Basin

Understanding hydrocarbon migration and accumulation mechanisms is one of the key scientif ic problems that should be solved for effective hydrocarbon exploration in the superimposed basins developed in northwest China. The northwest striking No.1 slope break zone, which is a representative of superimposed basins in the Tarim Basin, can be divided into five parts due to the intersection of the northeast strike-slip faults. Controlled by the tectonic framework, the types and properties of reservoirs and the ...     

Mechanism of diagenetic trap formation in nearshore subaqueous fans on steep rift lacustrine basin slopes—A case study from the Shahejie Formation on the north slope of the Minfeng Subsag,Bohai Basin,China

Diagenetic traps in conglomerate in nearshore subaqueous fans in the steep slope zones of rift basins have been important exploration targets for subtle reservoirs in eastern China. However, the mechanism of how those traps were formed is not clear, which inhibits further exploration for this type of reservoir. In order to solve the problem, we take as an example nearshore subaqueous fans in the upper part of the fourth member of the Shahejie Formation(Es4s) on the north slope of the Minfeng Subsag in the Dongying Sag. Combining different research methods, such as core observation, thin section examination, scanning electron microscope(SEM) observation, fluid-inclusion analysis, carbon and oxygen isotope analysis of carbonate cements, and analysis of core properties, we studied the genetic mechanisms of diagenetic traps on the basis of diagenetic environment evolution and diagenetic evolution sequence in different sub/micro-facies. Conglomerate in Es4 s in the north Minfeng Subsag experienced several periods of transition between alkaline and acidic environments as "alkaline–acidic–alkaline–acidic–weak alkaline". As a result, dissolution and cementation are also very complex, and the sequence is "early pyrite cementation / siderite cementation / gypsum cementation / calcite and dolomite cementation——feldspar dissolution / quartz overgrowth——quartz dissolution / ferroan calcite cementation / ankerite cementation / lime-mud matrix recrystallization / feldspar overgrowth——carbonate dissolution / feldspar dissolution / quartz overgrowth / pyrite cementation". The difference in sedimentary characteristics between different sub/micro-facies of nearshore subaqueous fans controls diagenetic characteristics. Inner fan conglomerates mainly experienced compaction and lime-mud matrix recrystallization, with weak dissolution, which led to a reduction in the porosity and permeability crucial to reservoir formation. Lime-mud matrix recrystallization results in a rapid decrease in porosity and permeability in inner fan conglomerates in middle-to-deep layers. Because acid dissolution reworks reservoirs and hydrocarbon filling inhibits cementation, reservoirs far from mudstone layers in middle fan braided channels develop a great number of primary pores and secondary pores, and are good enough to be effective reservoirs of hydrocarbon. With the increase of burial depth, both the decrease of porosity and permeability of inner fan conglomerates and the increase of the physical property difference between inner fans and middle fans enhance the quality of seals in middle-to-deep layers. As a result, inner fan conglomerates can be sealing layers in middle-to-deep buried layers. Reservoirs adjacent to mudstones in middle fan braided channels and reservoirs in middle fan interdistributaries experienced extensive cementation, and tight cemented crusts formed at both the top and bottom of conglomerates, which can then act as cap rocks. In conclusion, diagenetic traps in conglomerates of nearshore subaqueous fans could be developed with inner fan conglomerates as lateral or vertical sealing layers, tight carbonate crusts near mudstone layers in middle fan braided channels as well as lacustrine mudstones as cap rocks, and conglomerates far from mudstone layers in middle fan braided channels as reservoirs. Lime-mud matrix recrystallization of inner fan conglomerates and carbonate cementation of conglomerates adjacent to mudstone layers in middle fan braided channels took place from 32 Ma B.P. to 24.6 Ma B.P., thus the formation of diagenetic traps was from 32 Ma B.P. to 24.6 Ma B.P. and diagenetic traps have a better hydrocarbon sealing ability from 24 Ma B.P.. The sealing ability of inner fans gradually increases with the increase of burial depth and diagenetic traps buried more than 3,200 m have better seals.