Organic-rich formation and hydrocarbon enrichment of lacustrine shale strata: A case study of Chang 7 Member

Abstract

Lacustrine shale is an important target for the exploration of unconventional oil and gas in China beyond marine shale gas. However, the formation environment of lacustrine shale differs from that of marine shale, resulting in a different reservoir composition, organic matter, oil and gas content, and hydrocarbon mobility. In this study, the Chang 7 shale of the Yanchang Formation in the Ordos Basin was used to analyze the effect of volcanic activity on the paleoproductivity and preservation conditions during the formation of lacustrine shale. The results show that algae and bacteria were developed before the eruption. After the eruption, the number of bacteria declined, but the increased prosperity of algae reflects that the volcanic activity enhanced ancient productivity. The sulfate generated by volcanic activity promotes bacterial sulfate reduction, and the produced H 2 S leads to a strong reducing environment in the waterbody, which is conducive to the preservation of organic matter. Organic geochemical analysis shows that the black shale in the shale strata has a high total organic carbon (TOC) content and strong hydrocarbon generation potential, whereas the tuff has a low TOC content and can scarcely generate hydrocarbons, indicating that the tuff deposited by volcanic activity cannot be considered as effective source rock. In terms of storage space, shale is mainly laminar and dispersed, and it includes organic and inorganic pores. The development of organic pores is affected by thermal maturity, whereas inorganic pores mainly occur between detrital particles and crystals. Tuff is mainly supported by heterogeneous matrix and associated with alteration. Its pores include inter-and intragranular mineral pores. The development of tight sandstone pores is affected by compaction, cementation, and dissolution, which mainly consist of intra-and intergranular pores. The Chang 7 lacustrine shale generally contains oil, but different lithologies have different oil drainage efficiencies. Sandstone and shale exhibit the best and worst oil drainage efficiency, respectively. It is mainly affected by the pore size distribution, fluid properties, and rock wettability. Therefore, the development of shale oil should mainly focus on lacustrine shale formations with interbeds. The mutual dissolution of organic matter and hydrocarbons in the shale section leads to the poor mobility and difficult development of hydrocarbons.

Extremely High Primary Productivity and Organic Matter Enrichment in Lacustrine Sediments: Late Triassic Chang 73 Sub-Member, Western Ordos Basin

“…The black shales of the Chang 7 member in the Ordos Basin are generally characterized by a wide distribution area, large thickness, abundant organic matter, and suitable organic matter type and thermal maturity. Thus, it is regarded as the most significant oil and gas source for the entire Mesozoic oil reservoir [7,24]. The characteristics of source rocks vary in different regions due to the frequent changes in sedimentary facies in continental lacustrine basins.…”

“…Recent shale oil exploration breakthroughs on the Bakken Formation of the Williston Basin, Eagle Ford Formation of South Texas, and Barnett Formation of the Fort Worth Basin in North America have reformed the energy structure of the United States and had a profound impact on the world energy pattern [2][3][4][5]. China also pay attention on shale oil and gas resources and made successively discovering in the Permian Lucaogou Formation of the Junggar Basin [6], the Triassic Yanchang Formation in the Ordos Basin [7], the Jurassic Ziliujing Formation in the Sichuan Basin [8], the Cretaceous Qingshankou Formation in the Songliao Basin [9], and the Paleogene Shahejie Formation in the Bohai Bay Basin [10].…”

Hydrocarbon Generation Potential of Microbial Carbonate in the Western Qaidam Basin, China: Implication From Thermocompression Experiment

“…Moreover, when the temperature is over 340 °C, the expelled oil GP continues to increase, whereas the residual oil GP first enters a stable plateau and then decreases rapidly. The petroleum expulsion efficiency is a key parameter to evaluate the conventional and unconventional oil and gas resources (Cooles et al, 1986;Pepper and Corvi, 1995;Ritter, 2003;Eseme, 2007;Eseme et al, 2012;Liu et al, 2022). Cooles et al (1986) defined the petroleum expulsion efficiency as a measure of the extent of expulsion from a given source rock section.…”

Effect of Volcanic Activity on Hydrocarbon-Forming Organisms in Organic-Rich Shale: A Case Study of Dalong Formation in Northwestern Sichuan Basin, China

“…Previous studies and stratigraphic sections revealed the presence of multiple volcanic ash interlayers in the Dalong Formation. Deep fluid activities such as volcanic activity can significantly promote the formation of organic-rich shale and the proliferation of hydrocarbon-forming organisms (Liu et al, 2019a;Liu et al, 2019b;Liu et al, 2022). However, for the Dalong Formation high-quality source rocks, further efforts are needed to clarify the relationship between types of hydrocarbon-generating materials and volcanic ash interlayers.…”