Depositional characteristics of microbial carbonate reservoir and their effects on favorable reservoir distributions of the IV Member of Sinian Dengying Formation, Gaoshiti-Moxi area, Sichuan Basin, Southwest China

Abstract Previous scientific research on reservoirs of the Deng IV Member in Gaoshiti-Moxi area, the main production area of Anyue gas field with tens of billions of cubic meters reserves, concerns on karst paleogeomorphology reconstruction, facies distribution on platform margin and their effects on favorable reservoirs. However, the quality of microbial carbonate reservoirs is also closely related with their original depositional environments on the whole platform, which is significant for further petroleum exploration and exploitation of ultra-deep carbonate reservoirs in this area. Therefore, this paper attempts to reveal favorable microbial carbonate reservoir characteristics and sedimentary effects on their distribution and prediction, based dominantly on synthetic analysis of depositional characteristics, including both sequence stratigraphy and facies, such as seismic facies, microfacies and microfacies associations. The results show that favorable reservoirs of the Deng IV Member are classified into three types according to their space types, namely, fracture-cavity reservoir, pore-cavity reservoir and pore. Secondary dissolution pores and cavities are primary reservoir spaces developed mainly in non-skeletal grain dolomites with sparry cements, thrombolites, and stromatolites in microfacies association 1 (MA1), MA2, MA5, and MA7. Physical property of fracture-cavity type of reservoir is better in the upper and lower parasequence sets with a porosity between 1% and 5%, a permeability between 0.01 × 10 − 3 µm 2 and 1 × 10 − 3 µm 2 ; physical property of pore-cavity reservoir is fine throughout all the parasequence sets with a porosity between 2% and 4%, a permeability between 0.01 × 10 − 3 µm 2 and 1 × 10 − 3 µm 2 ; physical property of pore-cavity reservoir is the worst. In vertical, the favorable reservoirs are developed mainly in parasequence set 6 (PSS6) and PSS7 in vertical. On planar, they are distributed in well zone MX9-MX19-MX1 for the fracture-cavity type, MX105-MX110-GS20 for the pore-cavity type and MX17-MX107-MX41-MX102-GS102 for the pore type. What’s more, effects of depositional sequences, seismic facies, microfacies and microfacies associations on reservoir distribution indicate that to some extent, fracture-cavity type of reservoir is constrained by the top boundaries of PSS7, PSS2, parasequence 17 (PS17) and PS14; pore-cavity type of reservoir by the top boundaries of PSS7, PSS4, PS18 and PS12; pore type of reservoir by the top boundaries of PSS7, PSS6, PSS3, PSS2, PS18, PS17, PS14, and PS12. Seismic facies associated with shoals and of mound-flat complex are related with facies distributions of pore cavity and pore reservoirs. MA1, MA3, MA7, MA8 are predominant microfacies associations of favorable reservoirs of the Deng IV Member in Gaoshiti-Moxi area.