Reservoirs of Lower Cambrian microbial carbonates, Tarim Basin, NW China

The microbial carbonate reservoirs of the upper part of the Lower Cambrian Xiaoerbulak Formation have been researched based on outcrop section and thin section identification in the Aksu area, Tarim Basin. The microbialites are composed of peloid, lamination, thrombolite, stromatolite, dendrolite, oncolite and spongiostromata. There develop three types of microbial carbonate reservoirs: microbial reef, coated thrombolite and spongiostromata stromatolite dolomite. Two stages of microbial reefs are developed on paleokarst high in the Late Sinian: the first one with southward progradation is constructed by the Epiphyton, while the second one with northward transgression by cyanobacteria clots. The dendrolites, thrombolites and stromatolites in the reefs are potential for hydrocarbon reservoirs, with needle-like dissolved pores, thickness of 40 to 70 meters, lower porosity and lower permeability. Coated thrombolites are located on the paleo-depositional slope, characterized by the isolated dissolved pores, thickness of 30 meters, lower porosity and lower permeability. Spongiostromata stromatolite beds lie in the paleo-depositional depression, with layered fenestrules well developed, 18 meters thick, lower to medium porosity and lower permeability. The development of the microbial carbonate reservoirs are controlled by depositional paleo-topography, diagenesis and microbial structures.