MIGRATION TRAJECTORY, MICROFACIES AND RESERVOIR OF LACUSTRINE CARBONATE SHOAL—A CASE FROM LOWER JURASSIC, CENTRAL SICHUAN BASIN

The shelly limestones of Da’anzhai Member, Lower Jurassic are the main tight oil reservoirs in the Central Sichuan Basin. These limestones deposit in lacustrine carbonate shoals. However, the reservoirs are characterized by high heterogeneity. The purpose of this paper is to explain the migration trajectory of the carbonate shoals as a response to variable lake-level and its impact on distribution of sedimentary microfacies and reservoir heterogeneity, in order to predict favorable reservoir. Detailed information has been acquired by analyses of cores, rock thin sections, well-log data and producing test data.

The research work has brought about the following discoveries. Firstly, one third-order sequence, which each included one transgressive systems tract and one regressive systems tract, was defined. Moreover, seven parasequences and three parasequence sets were identified. The lake shoreline was positioned in each parasequence. Secondly, there were two microfacies of a carbonate shoal. Core was the core of a shoal, locating in high-energy setting reworked by waves. In contrast, margin was the edge of a shoal, or the transition between two cores in relatively low-energy setting. Thirdly, according to hydrodynamic properties, components and their contents of fillings between shells, and degree of recrystallization, four types of rocks in different microfacies had been identified: recrystallized shelly limestone, sparite shelly limestone, argillaceous shelly limestone and micrite shelly limestone. Fourthly, on the basis of reservoir properties of microfacies in different locations in one parasequence, it was indicated that favorable reservoirs deposited in shoal cores at the top of the parasequence, which consisted of sparite shelly limestone with dissolution pores and fissures and intercrystalline dissolution pores of calcites.

It is concluded that reservoir heterogeneity is controlled by its location within a sequence and sedimentary microfacies, and the favorable reservoirs are the sparite shelly limestones in shoal cores. Consequently, this paper predicts the favorable reservoirs by identifying sparite shelly limestones on well logs.