Orders of Sequence Boundaries and Its Control on Diagenesis of Bioclastic Limestone Reservoir

Abstract

AbstractDuring the deposition of bioclastic limestone of Mishrif Formation in the Middle East, H oilfield was in a warm and humid climate environment, and the atmospheric fresh water dissolution was very active at the sequence boundaries. Based on the data of core, well-logging, cast thin section and experimental analysis, taking the MB2-MC1 section of H oilfield as an example, the order identification of sequence boundaries is carried out, and the control mechanism of different orders of sequence boundaries on the diagenesis of bioclastic limestone reservoir is studied. The results show that MB2-MC1 section is developed in SQ2 third-order sequence, including four fourth-order sequences, in which multiple fifth-order and sixth-order sequences are developed. With the increase of exposure time at the sequence boundaries, the dissolution intensity increases, the pore radius of the reservoir enlarges and the connectivity improves. At the sixth-order sequence boundary, the aragonite and high magnesium calcite bioclastic particles are preferentially dissolved by atmospheric fresh water, resulting in selective dissolution, forming the reservoir dominated by intragranular dissolved pores and mould pores. At the fifth-order sequence boundary, the unstable bioclastic particles are selectively dissolved first. A certain degree of non-selective dissolution occurs after the stabilization of minerals, forming the reservoir with mainly mould pores and a small amount of intergranular dissolved pores. At the fourth-order sequence boundary, a large amount of fresh water infiltrates along the pore system, causing non-selective dissolution, forming the reservoir dominated by intergranular dissolved pores. The third-order sequence boundary undergoes the longest exposure time, goes through the most adequate dissolution of atmospheric fresh water and strong non-selective dissolution occurs, forming the reservoir with mainly intergranular dissolved pores and a small amount of dissolved vugs, which is also the best reservoir of the Mishrif Formation. The quality and distribution characteristics of bioclastic limestone reservoir can be predicted and evaluated by fine order identification of sequence boundaries.KeywordsOrder of sequence boundaryBioclastic limestoneDiagenesisReservoir characteristics