Reservoir Potential of Silurian Carbonate Mud Mounds in the Southern Sichuan Basin, Central China

Lower Silurian mud mounds of the Shinuilan Formation,located in the southern Sichuan Basin,China,have developed in open shelf settings in deeper water than shallow-water reef-bearing limestones that occur in the region.An integration of the outcrop,drill data and seismic profiles show that contemporaneous faults have controlled the boundary and distribution of the sedimentary facies of Lower Silurian rocks in the southern Sichuan Basin.Mounds appear to have developed in the topographic lows formed by synsedimentary faulting,on the shelf of the Yangtze Platform.Average mound thickness is 20 m,maximum 35 m.Mounds are composed mainly of micrite,possibly microbially bound,and are overlain by shales.Mound tops are preferentially dolomitized,with the Mg2+ source probably from the clay content of the mound-top carbonate.Microfacies analysis and reconstruction of the diagenetic history reveal that the mound tops have higher porosity,and are gas targets;in contrast,mound cores and limbs show pores filled by three generations of calcite cement,and therefore have a low gas potential.