Wenzhi Zhao

Geological conditions and distributional features of large-scale carbonate reservoirs onshore China

Abstract Based on well cores and thin section observations of more than 300 wells from major exploration target areas and formations in the Tarim, Sichuan and Ordos Basins, combined with seismic, well logging and testing data, the types and characteristics of carbonate reservoirs as well as the geologic conditions for their extensive development are analyzed systematically, and their distribution features are summarized. All varieties of marine carbonate reservoirs are developed in China, including three types of large-scale effective reservoirs, which are (1) depositional reef-shoal and dolomite reservoirs, (2) epigenetic dissolution-percolation reservoirs and (3) deep burial-hydrothermal altered reservoirs. Besides sedimentary facies, paleoclimate and paleogeomorphy, other factors controlling the development of deep large-scale effective reservoirs include interstratal and intrastratal dissolution-percolation and burial dolomitization which can be impacted by hydrothermal processes. Large effective reservoirs in deep carbonate rocks are distributed along unconformities and hiatuses in sedimentation, while reservoirs of epigenetic dissolution-percolation type extend from paleohigh uplift zones to lower slope reliefs. The reservoirs are widely distributed in stratified planar forms, and are superposed by multi-stage karstification processes vertically and have obvious heterogeneity controls. Burial dolomitization is restricted by primary sedimentary facies, and can form extensive effective reservoirs in deep layers in layered or stratified shapes. Hydrothermal related reservoirs are always distributed along deep, large faults, forming effective reservoirs in the form of a bead string in vertical direction and band-rod horizontally, which are not restricted by burial depth.

Geological features and evaluation techniques of deep-seated volcanics gas reservoirs, Songliao Basin

Abstract The significant breakthrough of natural gas exploration in deep-seated volcanic rocks of Songliao Basin, China, demonstrates that the volcanic rocks as reservoirs can form natural gas accumulations where excellent source–reservoir–caprock assemblages exist. This paper reveals several features of volcanic gas accumulations: (1) Two types of gas reservoirs, including hydrocarbons and non-hydrocarbons, are developed; the former is organic and usually trapped in shallower volcanics associated with sub-deep-seated faults, while the latter is mainly inorganic and accumulated in deep-seated volcanics associated with large-scale deep-cut basement faults. (2) Lower Cretaceous Shahezi Formation coal series source rocks, Yingcheng Formation volcanics, and Denglouku Formation mudstone form good source–reservoir–seal associations that control the major volcanic plays. (3) Volcanic crater and eruptive facies spread along deep-seated faults, so discordogenic faults control the distribution of large-scale volcanic gas reservoirs. (4) Fracture zones, accompanied with large-scale faults, control the distribution of high-yield zones. (5) Effusive volcanic rocks in the deep parts of sags have huge thickness and touch extensively with source rocks, so the gas accumulation potential is promising if reservoirs exist. Evaluation techniques can be summarized in three steps: (1) Determining the distribution of volcanic rocks by joint inversion of gravitational, magnetic, and seismic data. (2) Predicting the distribution of volcanic reservoirs by multiple methodologies. (3) Predicting the distribution of gas-bearing volcanic rocks by integrated methods, including gas detention.

Geological difference and its significance of marine shale gases in South China

Abstract Organic-rich marine shales are developed in both the Cambrian Qiongzhusi Formation and the Ordovician Wufeng Formation–Silurian Longmaxi Formation in South China, but are different in the drilling results of shale gas exploration. Comparing the differences in shale gas formation conditions between Qiongzhusi and Wufeng-Longmaxi has practical and theoretical significance. This study reveals: (1) in the Sichuan Basin, the Wufeng-Longmaxi Formation has slightly higher TOC than the Qiongzhusi Formation, whereas Qiongzhusi Formation has some local high TOC areas outside of the Sichuan Basin; (2) the Qiongzhusi Formation has much higher thermal evolution degree than the Wufeng-Longmaxi Formation; (3) with undeveloped organic pores, the Qiongzhusi Formation has a 1/3 to 1/2 porosity of the Wufeng-Longmaxi Formation; (4) Qiongzhusi shale has a lower gas content, only 1/2 of that in Wufeng-Longmaxi shale; (5) the Qiongzhusi Formation is mainly composed of siliceous shale and the silica is hot water origin, whereas the Wufeng-Longmaxi Formation consists mainly of calcareous siliceous shale and the silica is biogenic origin; (6) the Wufeng-Longmaxi Formation has overpressure, while the Qiongzhusi Formation is normal in pressure. The reasons for the differences are: (1) different sedimentary environments affect TOC and shale thickness; (2) the Qiongzhusi Formation is over-mature, which caused depletion of hydrocarbon generation, organic carbonization, porosity reduction, and gas content drop; (3) the bad roof and floor conditions of the Qiongzhusi Formation are not good for shale gas preservation; (4) Wufeng-Longmaxi Formation is located in the slope and syncline accompanied with overpressure, and is in favor of preservation and high production of shale gas; (5) the uranium content in the Qiongzhusi Formation is twice that of the Wufeng-Longmaxi Formation, which is the main reason of its higher thermal evolution degree. It is concluded that shale gas enrichment in the marine shale in South China requires favorable geological conditions: organic-rich intervals, moderate thermal evolution, rich organic pores, high gas content, good roof and floor preservation conditions, and moderate burial depth. The Wufeng-Longmaxi Formation has better shale gas enrichment conditions and higher resource potential, whereas the Qiongzhusi Formation has poorer shale gas accumulation conditions and limited favorable areas.

The multi-staged “golden zones” of hydrocarbon exploration in superimposed petroliferous basins of onshore China and its significance

Abstract The hydrocarbon generation model proposed by Tissot et al. points out the temperature and depth of “liquid HC window”, which has become a “golden zone” for hydrocarbon exploration. It has been proved by exploration that multi-staged “golden zones” for hydrocarbon exploration is commonly developed in the superimposed petroliferous basins of onshore China. There are three factors for the formation of multi-staged “golden zones” of HC exploration in the superimposed basins: (1) source kitchens developed with multi-periods and multi-centers which have verified to lead multi-stages of HC generation with large scale, (2) multi-staged reservoirs develop with large scale, (3) hydrocarbon accumulations occur with the multi-periods and late effectiveness. The conventional source kitchens, dispersed liquid HC-cracking gas kitchens and effective reservoirs with large scale join together to control the distribution of “golden zones” in timing and space. Explorational “golden zones” have the characteristics of inherited stacking and lateral variation. Paleo-highs, paleo-slopes, paleo-platform margins, and multi-period inherited fault zones control the distribution of hydrocarbons in the explorational “golden zones”. The concept of multi explorational “golden zones” helps to deepen the knowledge of new hydrocarbon distributional regularity which revealed recently in China. It shows that there exist economic resources in the deep section of the superimposed basins of onshore China. The hydrocarbon discovery history in the superimposed petroliferous basins has the feature of multi-peaks of proven reserve increase and lasting a quite long, which indicates a huge potential for future exploration.

Further discussion on the connotation and significance of the natural gas relaying generation model from organic matter

Abstract The natural gas relaying generation model (abbreviated as relay model) of organic matter focuses on the potential of gas generation and accumulation derived from the dispersed liquid hydrocarbons retained in source rocks during high to over mature stage (Ro>1.6%). It means the gas generation converts from thermal degradation of kerogen to oil cracking with the increase of thermal maturation, and it forms relaying gas generation processes at different thermal maturation. According to the supplementary kinetic studies and anatomization of discovered gas reservoirs, it was found that source rocks still contain a large amount of liquid hydrocarbons within oil window even after expulsion, which can serve as the effective gas source kitchen during the high to over mature stage. The maturity for the gas largely generated from oil cracking is greater than 1.6% vitrinite reflectance (Ro), and the content of methyl cyclohexane can be utilized as a specific index for identification of such natural gas. Conventional outside source natural gas reservoirs with considerable scale could be formed due to the existence of migration pathway, driving force and process for effective expulsion under high diagenetic environment. The significance of the natural gas relay model is reflected at the following three aspects: (1) focusing on natural gas accumulations of dispersed liquid hydrocarbons retained in source rocks, (2) determination of the main gas generation periods of retained liquid hydrocarbons, and (3) evaluation on the conventional gas accumulation potential of the cracking gas outside of the source kitchen from the detained liquid hydrocarbons during the high to over mature stage.

Assessment on gas accumulation potential and favorable plays within the Xu-1, 3 and 5 Members of the Xujiahe Formation in the Sichuan Basin

Abstract Natural gas exploration and discovery of hydrocarbons in the Xujiahe Formation in the Sichuan Basin are mainly concentrated in the Xu-2, 4 and 6 Members. The discovered gas reservoirs are characterized by a low filling degree, high water saturation in lower intervals, low levels of produced reserves and a limited number of gas-bearing reservoirs. The Xu-1, 3 and 5 Members are identified as source rocks. Based on the study of natural gas accumulations and several gas production wells in the Xu-1, 3 and 5 Members, this paper proposes that large scale sand bodies are developed in the Xu-1, 3 and 5 Members in the northwest, southwest and central parts of the Sichuan Basin. In each member, the favorable gas bearing area is about 5 000-11 000 km2, the reservoir rocks have good continuity with single layer extensions of up to 15-20 km. It is believed that these sand bodies have favorable conditions for gas accumulation within source kitchens to form lithologic or structural-lithologic gas reservoirs and have better accumulation potential and higher gas-bearing abundance than that of the Xu-2, 4 and 6 Members. Exploration practices have revealed that economic gas reserves may be generated. It is suggested that more efforts be made to study the gas accumulation conditions in the Xu-1, 3 and 5 Members (especially the Xu-3 and Xu-5 Members) and more exploration work be conducted in these strata.

Types, characteristics, origin and exploration significance of reef-shoal reservoirs: A case study of Tarim Basin, NW China and Sichuan Basin, SW China

Abstract Taking reef-shoal bodies developed in multiple formations in the Tarim and Sichuan Basins as examples, this paper examines the origin of pores and heterogeneity of different types of reef-shoal reservoirs, and tries to find out the distribution rules of various reef-shoal reservoirs. Reef-shoal reservoirs can be classified into two main types, i.e. biohermal reservoirs and grain shoal reservoirs. The former, related to reef framework, mostly in the form of frilled platform margins, can be subdivided into two categories; interior platform reef complex and margin reef complex (sedimentary or tectonic origin). The latter, unrelated to the reef framework, is controlled by the fluctuation of sea level on gentle slope carbonate platforms, are widely developed on carbonate ramps due to the migration of shoals, and can be subdivided into interior platform shoals and platform-margin shoals on steep slopes. Reservoir space within reef-shoal carbonates was primarily formed during syngenetic and burial stages. Syngenetic porosity is controlled by the original rock fabric and dissolution related to the exposure of sequence boundaries, while porosity development and distribution during the burial stage is closely related to that in the syngenetic stage, showing inherited features. Reef-shoal reservoirs are strongly heterogeneous, and complicated with respect to effective reservoir distribution. Platform margin reefs and biohermal shoals coexisting with reefs on the top of shallowing upward sequences are the main reservoirs and rich in porosity. The grain shoals deposited on top of shallowing upward cycles of isolated reefs are also the main zones rich in pores, and usually interbed with tight reservoirs. Overall, grain shoal reservoirs are larger in scale and better in physical properties than biohermal reservoirs.

Principal characteristics and forming conditions for medium-low abundance large scale oil/gas fields in China

Abstract In recent years, some of large scale oil or gas fields with medium-low abundance have been discovered in onshore China. They have such characteristics in common as low porosity and permeability, small oil or gas column, low reserves abundance, and large scale. Studies conducted by the authors show that the formations of large-area reservoirs with medium-low abundance have the following favorable conditions: the large scale sandbodies caused by drag flow in the hinterland of large lacustrine basins, alternating broadly with source rocks like “sandwich-type” architecture of reservoir-source rock association, provide fundamental basis of oil/gas accumulation in large scale; the small oil or gas column and normal to low pressure system reduce the restrict requirement on cap-rock quality, which leads to large scale reservoir formation; the strong reservoir heterogeneity with poor connectivity minimizes gas escape energy and ensures wide oil/gas accumulations even in the area with relatively poor geological conditions; the late uplifting and unloading environment makes it possible for source rock desorb and discharge hydrocarbon to form reservoirs with young generation. Studies indicate that there are nice prospects in discovering large scale lithological reservoirs in the synclinal areas of large terrestrial depression basins of China. This concept has guided the oil/gas discoveries in synclinal areas of Ordos, Songliao, and Sichuan Basins, promoted the great increase of oil and gas reserves and shown a good potential for future exploration.

Oil cracking: An important way for highly efficient generation of gas from marine source rock kitchen

The potentials of gas generation by kerogen in the late period and by crude oil cracking are closely related to the origin of natural gas in the high-to over mature marine area and their exploration perspectives. The carbon structure of kerogens, with different types and at different evolution stages, have been experimentally studied using the high magnetic field solid13C nuclear magnetic resonance technique in order to determine the oil and gas potential of kerogens. Results show that the contents of gas potential carbon(GPC) of types I, II, III kerogens at the high-to over mature stage are very low, indicating their weak gas-generating capacity and limited gas production; however, the content of oil potential carbon(OPC) of the low mature type I kerogen is much higher, implying that a large amount of crude oil generated during the oil-generating period will be the material for later gas generation by oil cracking. The kinetic experiment of gas generation by crude oil cracking shows that, when the temperature is about 160°C (Ro=l.6%), the crude oil will start to produce large amounts of gas; the temperature range for major gas generation of crude oil is higher than that of the kerogens, and the gas production is 2 to 4 times higher than that of kerogens. The natural gas derived from oil cracking (called oil-cracked gas) is much abundant in methyl hexamethylene, which is quite different from the natural gas produced by thermal degradation of kerogens (named kerogen degradation gas) at high-to over mature stage.

Similarities and differences between natural gas accumulations in Sulige gas field in Ordos basin and Xujiahe gas field in Central Sichuan Basin

Abstract Both Sulige gas field in the Ordos Basin and Xujiahe gas field in Central Sichuan Basin are large low-porosity and low-permeability gas fields composed of conventional gas reservoirs and unconventional gas accumulations, representing typical cases of low-grade natural gas accumulations in China. Studies show there are both similarities and differences between these two fields in terms of forming conditions, mechanism and distribution. The similarities include: (1) large-scale development of accumulation conditions and close source-reservoir contacts, providing basis for large-scale accumulations; (2) large-scale energy storage during source kitchen burial and large-scale gas expulsion during uplift, providing important conditions for large-scale natural gas accumulation; (3) coexisting volume flow and diffusion flow, giving rise to the co-existence of conventional gas reservoirs and unconventional gas accumulations; and (4) clustered lithologic gas accumulations on a large scale. The differences lie in: (1) different source-reservoir-seal assemblages, giving birth to two kinds of accumulations, i.e. large area accumulations and large-scale patchy accumulations; (2) different source kitchen continuity and gas supply potentials, leading to different enriched gas-bearing layer series and different abundances. Timely understanding of the similarities and differences between large-scale natural gas accumulations with low porosity and permeability in these two basins would be conducive to avoid risks and improve exploration success ratio and benefits.