Early development and late preservation of porosity linked to presence of hydrocarbons in Precambrian microbialite gas reservoirs within the Sichuan Basin, southern China

摘要

The ancient Ediacaran Dengying Formation consists of microbialites with unusually high porosity that represent a large proportion of the Weiyuan-Anyue gas reservoirs in southwestern China's Sichuan Basin. However, a clear model for the development and preservation of these microbialite reservoirs has yet not been determined. We used samples from outcrops and borehole cores to characterize these microbialites and propose a model for early development and subsequent reservation of their porosity. Primary porosity in the microbialite framework was mainly represented by fenestrate vugs and elongated cavities between microbial layers that were formed during initial sedimentation as well as secondary karst vugs and caves formed by meteoric karst resulting from subsequent tectonic uplift; these features represent the early development of the microbialite reservoirs. Subsequent filling of vugs by early-stage radial dolomite (RD) and late-stage saddle dolomite (SD) occurred during the deep burial stage, with delta O-18 and homogenization temperature patterns suggesting continuous cementation. However, most vugs were not cemented due to combination of (1) a rigid dolomitized framework, (2) a closed burial system restricting the flow of mineralized fluid, (3) the presence of petroleum or pyrobitumen, and (4) acidic fluid conditions related to high concentrations of H2S and CO2. The third and fourth factors were linked to presence of hydrocarbons and had made big contribution to porosity retention during deep burial diagenetic stage.