Oil & Gas Geology ›› 2023, Vol. 44 ›› Issue (1): 55-74.doi: 10.11743/ogg20230105

• Petroleum Geology • Previous Articles     Next Articles

Diagenesis and porosity evolution of microbial carbonate rocks undergone a deep burial history: Taking the Leikoupo Formation of Middle Triassic in western Sichuan Basin as an example

Yixiong QIAN1(), Hengzhi WU2, Lingfang ZHOU1, Shaofeng DONG3, Qiongxian WANG4, Xiaobo SONG4, Meizhou DENG4, Yong LI4   

  1. 1.Wuxi Branch,Petroleum Exploration and Production Research Institute,SINOPEC,Wuxi,Jiangsu 214126,China
    2.International Petroleum Exploration and Production Corporation,SINOPEC,Beijing 100029,China
    3.State Key Laboratory of Oil and Gas Reservoir Geology and Exploration,Southwest Petroelum University,Chengdu,Sichuan 610500,China
    4.Institute of Exploration and Development,Southwest Oil and Gas Company,SINOPEC,Chengdu,Sichuan 610081,China
  • Received:2022-05-11 Revised:2022-08-15 Online:2023-01-14 Published:2023-01-13

Abstract:

The microbial dolomite reservoir in the Middle Triassic Leikoupo Formation, western Sichuan Basin, has been one of the exploration targets with a buried depth excessing 5 000 m and it serves as an excellent object of study for diagenetic fluids and pore evolution that are probably accountable for the formation of deeply-buried microbial carbonate reservoirs. Samples and data from more than 10 wells and several outcrops and seismic sections were studied with multi-technical methods, such as cast thin section observation, cathodoluminescence microscopy (CL), scanning electron microscopy+energy-dispersive X-ray spectroscopy (SEM-EDS), fluid inclusion microthermometry, carbon and oxygen isotope analysis via microsampling, carbonate clumped isotopes thermometer (△47), U-Pb dating of calcite, FIB-SEM and so on. The results show that there are three early diagenetic subsystems: open, closed and semi-open (transition), which respectively correspond to the intensive meteoric water influx in algal dolostone in intertidal-subtidal zones under humid climate, the weak meteoric water influx in evaporate rock-algal lamellated micritic dolostone in supratidal-intertidal zones under arid climate and the interaction between bacteria & microbial and carbonate mud druing an early supergene-shallow burial period. Large-scale dolomitzation and de-dolomitization during the burial-tectonic epochs are believed to take place repecitvely in the Carnian(226.50 ± 9.68 Ma)and Norian(211.50 ± 1.50 Ma)stages of Late Triassic, with temperatures of 43 ℃ to 54 ℃ and 50 ℃, respectively. Four to five successive dolomitization and calcite cementation stages have been identified, showing that the δ18Owater (PDB‰) of parent fluids varies between -0.83 ‰ and 9.70 ‰ for dolomite, and between -1.16 ‰ and 12.94 ‰ for calcites, while with the enlargement of cement crystalline, the overall δ18Owater slowly reduces, indicating an increase in temperature and fluctuationg salinity. Statistics reveal that vugs and solution-enlarged fractures (43.69 %), framework pores (32.38 %) and micro-poros are the major pore types of microbial carbonate rocks. The micropores are well connected as organic acid-rich pore fluid inhibiting large-scale cementation and result in the partial preservion of pores with a good connectivity. Thereby, the shallow subtidal to supratidal zones with a development of shallow-up Mirokovich sedimentary cycle for microbial carbonate rocks, combined with the open to semi-open early diagenetic system, are the basis of reservoir porosity development and preservation, and the superposition and transformation of burial diagenetic fluids leads to the overall reduction of porosity.

Key words: pore evolution, diagenetic fluid, microbial carbonate rocks, Leikoupo Formation, Middle Triassic, western Sichuan Basin

CLC Number: