Oil & Gas Geology ›› 2025, Vol. 46 ›› Issue (3): 759-776.doi: 10.11743/ogg20250305

• Petroleum Geology • Previous Articles     Next Articles

Types and distributions of endogenetic components in fine-grained sediments and their implications for the evaluation of shale oil and gas sweet spots

Zhensheng SHI1,2(), Tianqi ZHOU1,2(), Pengfei WANG1,2   

  1. 1.Research Institute of Petroleum Exploration & Development,PetroChina,Beijing 100083,China
    2.National Energy Shale Gas R&D (Experiment) Center,Langfang,Hebei 065007,China
  • Received:2025-03-06 Revised:2025-04-29 Online:2025-06-30 Published:2025-06-26
  • Contact: Tianqi ZHOU E-mail:shizs69@petrochina.com.cn;zhoutianqi@petrochina.com.cn

Abstract:

Efficient exploration and exploitation of shale oil and gas depend heavily on the accurate prediction of hydrocarbon sweet spot distribution. Meanwhile, the spatiotemporal differentiation and coupling of endogenetic components in fine-grained sediments produce a key effect on sweet spot distribution. Through an in-depth analysis of typical oil and gas-bearing shales both at home and abroad, we systematically reveal the types and distribution patterns of endogenetic components in fine-grained sediment, as well as the mechanisms by which these components control shale oil and gas sweet spots. The results indicate that the major endogenetic components include biogenic silica, biogenic carbonate minerals, biogenic phosphate minerals, authigenic organic matter, and intraclasts. These components are primarily concentrated in zones that terrigenous clastics pass by, zones that are difficult to reach for terrigenous clastics, areas influenced by upwelling ocean currents, and nearshore areas experiencing rapid transgression. The dissolution-reprecipitation process of biogenic silica, among others, leads to the formation of authigenic quartz, which inhibits organic matter degradation through hydrogen bonding. While the biogenic carbonate and phosphate mineral cementation maintains reducing environments by decreasing pore water mobility. All these are conducive to organic matter enrichment. The biogenic silica in endogenetic component-rich shales, through its synergy with organic matter, contributes to the development of pores and fractures, which in turn leads to the formation of a distinctive organic pore-microfracture coupling structure. Furthermore, the rigid support and pore networks provided by the biogenic silica frameworks provide, along with swelling stress and dissolution induced by the thermal evolution of organic matter, collectively drive the formation of a three-dimensional microfracture system. Endogenetic components play a determinant role in the distribution of shale oil and gas sweet spots by influencing organic matter enrichment, pore structure, gas adsorption, seepage characteristics, and rock brittleness. Accordingly, shale oil and gas sweet spots generally feature high endogenetic component content, elevated nano-scale porosity, abundant microfractures, and high brittle mineral content. Presently, there is a need for further investigation into the genesis and enrichment mechanisms of endogenetic components in fine-grained sediments, as well as the quantitative relationships of these components with the physical properties and brittleness index of reservoirs. Moreover, the mechanisms underlying the dynamic controlling effects of the endogenetic components on organic matter preservation and pore evolution during diagenesis warrant further exploration. In the future, it is necessary to further identify and analyze the endogenetic components using methods such as multi-scale characterization, artificial intelligence (AI), and interdisciplinary integration, with a view to supporting the efficient exploitation of shale oil and gas.

Key words: organic matter, biogenic silica, carbonate mineral, endogenetic component, sweet spot distribution, fine-grained sediment, shale oil and gas

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