Pyrobitumens in the Lower Triassic Gas Reservoirs of the Puguang and Maoba Gas Fields, Sichuan Basin, China: Implications for Oil Cracking and Thermochemical Sulfate Reduction
Pyrobitumens are common in deep carbonate gas reservoirs of the Sichuan Basin. Incident light microscopy and SEM-EDS have been used to study the origin of pyrobitumens from the Lower-Middle Triassic carbonates in northeastern Sichuan Basin. Pyrobitumen reflectance properties, coke microtextures and relative S/C ratios are used to classify the pyrobitumens. In the Middle Triassic reservoirs of the Leikoupo Formation, pyrobitumens with low-reflecting isotropic to fine-grained mosaic coke microtextures commonly occur together with unaltered anhydrite, indicating NSO-enriched, low aromaticity crude oils with relatively little oil to gas transformation. In contrast, pyrobitumens with high bireflectance and very anisotropic coarse flow to domain cokes dominate in the Lower Triassic oolitic carbonates of the Feixianguan-Changxing formations. The highly deformed pyrobitumens with fibrous coke textures and high bireflectance suggest the oil to gas cracking within some of the reservoir units is associated with the development of overpressured reservoirs. The variations in the dominant pyrobitumen types from the southwestern to northeastern Puguang gas field are attributed to increased crude oil alteration during thermal cracking and thermochemical sulfate reduction, leading to increased reflectance and S/C atomic ratio and decreased O/C atomic ratio of pyrobitumens. The interactions between crude oil and sulfur-enriched, highly alkaline fluids is suggested by the presence of high S/C, very fine-grained, late stage calcined needle cokes within brecciated pyrobitumens. The presence of highly heterogeneous and largely unaltered reservoir bitumens, together with their interplay with natural gases of variable hydrogen sulfide contents, indicates that thermochemical sulfate reduction in this area has been controlled by the spatial relationship with paleo-oil/water contact and limited by sulfate concentrations in the reservoirs, a critical factor in controlling the chemical and isotope compositions of the gaseous alkanes in the reservoirs.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009