Microfacies of lacustrine high-energy ramp carbonate shoal and their control on reservoir—a case from Lower Jurassic, Central Sichuan Basin

Microfacies of lacustrine high-energy ramp carbonate shoal and their control on reservoir—a case from Lower Jurassic, Central Sichuan Basin TIANSHU ZHANG, XIAOPING YANG, YINYE WU, SHIZHEN TAO, ZHENGLIAN PANG, RONGCHANG FENG,... [ view full abstract ]

Microfacies of lacustrine high-energy ramp carbonate shoal and their control on reservoir—a case from Lower Jurassic, Central Sichuan Basin
TIANSHU ZHANG, XIAOPING YANG, YINYE WU, SHIZHEN TAO, ZHENGLIAN
PANG, RONGCHANG FENG, HONG ZHANG
Research Institute of Petroleum Exploration & Development，CNPC
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 high-energy ramp carbonate shoals. However, the reservoirs are characterized by high heterogeneity, with low porosity and permeability, complex pore and throat structures, and abundant microfractures. The purpose of this paper is to explain the impact of sedimentary microfacies on heterogeneity, in order to provide scientific basis for favorable reservoir prediction. Detailed information has been acquired by analyses of outcrops, cores, rock thin sections, scanning electron microscope, and producing test data. The research work has brought about the following discoveries. Firstly, there were two microfacies of a carbonate shoal: core and margin. 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. Secondly, 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. Thirdly, on the basis of reservoir properties of different microfacies, it was indicated that favorable reservoirs deposited in shoal cores, which consisted of sparite shelly limestone with dissolution pores and fissures and intercrystalline dissolution pores of calcites. Fourthly, rock thin section-to-well log calibrating revealed that a low gamma ray response of less than 30 API was the characteristic feature of sparite shelly limestones. Subsequently, based on gamma ray log of one hundred and six wells, isopach mapping of sparite shelly limestones was showed. It is concluded that reservoir heterogeneity is controlled by 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.
Keywords: lacustrine high-energy ramp, carbonate shoal, sedimentary microfacies, shelly limestones, reservoir.