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Abstract

DOI: 10.1306/05111817021

Diagenesis and its impact on a microbially derived carbonate reservoir from the Middle Triassic Leikoupo Formation, Sichuan Basin, China

Lei Jiang,1 Suyun Hu,2 Wenzhi Zhao,3 Zhaohui Xu,4 Shuyuan Shi,5 Qilong Fu,6 Hongliu Zeng,7 Wei Liu,8 and András Fall9

1Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, 19 Beitucheng West Road, Chaoyang District, Beijing 100029, China; Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, 10100 Burnet Road, Austin, Texas 78713; Institutions of Earth Science, Chinese Academy of Sciences, No. 19 Beitucheng West Road, Chaoyang District, Beijing 100029, China; [email protected]
2Research Institute of Petroleum Exploration and Development, PetroChina, 20 Xueyuan Road, Haidian District, Beijing 100083, China; [email protected]
3Research Institute of Petroleum Exploration and Development, PetroChina, 20 Xueyuan Road, Haidian District, Beijing 100083, China; [email protected]
4Research Institute of Petroleum Exploration and Development, PetroChina, 20 Xueyuan Road, Haidian District, Beijing 100083, China; [email protected]
5Research Institute of Petroleum Exploration and Development, PetroChina, 20 Xueyuan Road, Haidian District, Beijing 100083, China; [email protected]
6Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, 10100 Burnet Road, Austin, Texas 78713; [email protected]
7Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, 10100 Burnet Road, Austin, Texas 78713; [email protected]
8Research Institute of Petroleum Exploration and Development, PetroChina, 20 Xueyuan Road, Haidian District, Beijing 100083, China; [email protected]
9Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, 10100 Burnet Road, Austin, Texas 78713; [email protected]

ABSTRACT

The uppermost Middle Triassic Leikoupo Formation in the western Sichuan Basin of China has recently been shown to host as much as 5.3 tcf (1.5 × 1012 m3) of natural gas resources. The reservoir rocks, composed mainly of microbially derived dolomudstone (e.g., thrombolites and stromatolites), are characterized by low porosity (<8%) and permeability (<0.001 to 10 md). The limestone is commonly tight and not of reservoir quality because of abundant meteoric calcite cementation, whereas the dolostone has various types of pores dominated by solution-enlarged pores and vugs, microbial framework pores, and micropores. Breccias are well developed in places, probably because of dissolution of underlying evaporites (e.g., anhydrite) by an influx of low-salinity fluids (e.g., freshwater and seawater) during an early burial stage. Early dolomitization created micropores in the dolomudstone, and subsequent diagenetic events were dominated by calcite, dolomite, quartz cementation, pyrite replacement, compaction, fracturing, and development of stylolites. Localized hydrothermal activity has been evidenced by high homogenization temperatures (∼160°C–200°C) obtained from fluid inclusions in fracture-filling cements. Bacterial sulfate reduction probably resulted in H2S generation, pyrite precipitation, and solution-enlarged pore and vug formation, whereas part of the current H2S in these reservoirs may have been sourced from thermochemical sulfate reduction or an underlying formation (e.g., the Feixiangguan Formation). Development of microfractures and associated micropores was probably the final diagenetic event, which improved pore interconnectivity. This study confirms the effect of diagenesis on the development of a microbial dolomudstone reservoir, which may be applicable to other similar microbial carbonate reservoirs elsewhere, for example, Middle Triassic sections of the Tethys region and offshore Brazil.

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