CONTROL OF SEDIMENTARY FACIES AND HIGH-FREQUENCY SEQUENCES ON THE RESERVOIR OF LONGWANGMIAO FORMATION IN MOXI AREA, CENTRAL SICHUAN BASIN

Four sedimentary subfacies of lagoon, inter-shoal sea, grain shoal and dolomitic flat of the Longwangmiao Formation in Moxi area, Central Sichuan Basin are identified by study on the sedimentary facies, high-frequency sequences and reservoir genesis base on the drill cores, thin sections and logging data. The Formation can be divided into two fourth-order sequences while each includes three to five fifth-order sequences separately. The porosity of reservoirs is generally good with the main reservoir space as intercrystalline pores, solution caves and fissures, the main reservoir rocks being finely crystalline, silty and grained dolomites. The favorable reservoirs are obviously controlled by sedimentary facies and high-frequency sequences. Grain shoals and dolomitic flats are the most porous reservoir facies. The HST in the fourth-order sequences usually shows better reservoir quality than the TST. Early stage dissolution developed near the fourth and fifth order sequence boundary plays the key role on the porosity improvement of dolomitic flats and grain shoals and also provides basis for the later stage burial dissolution and telogenetic karstification.