Influence of multi-stage volcanic events on the Late Cretaceous-Paleogene reservoirs and its geological significance in the northern Central Myanmar Basin

The northward subduction of the Neo-Tethys oceanic crust triggered multiple magmatic activities in the West Myanmar Arc,which in turn influenced the deposition of sedimentary pyroclastic rocks from the Cretaceous to Eocene strata in the Central Myanmar Basin(CMB).The pore structure of these lithologic reservoirs is complex and rich in tuffaceous sandstone,which plays an adverse role in reservoir development in this region.To understand the development characteristics and genetic mechanism of the pyroclastic rocks within three sets of reservoirs in this area,a comprehensive analysis was conducted through borehole core observations,thin section identification,scanning electron microscope analysis,and mercury injection tests.The tuffaceous sandstone from the upper Cretaceous to the Eocene is dominated by intermediate-acid volcanic rock debris.The pyroclastic rocks exhibit evident chloritization and ironization,with residual intergranular pores being the principal type accompanied by a smaller amount of intergranular dissolved pores and intragranular dissolved pores.The highest porosity is observed in the Eocene tuffaceous sandstone,ranging from 8%to 12%.The Late Cretaceous to Paleocene sandstones exhibit lower porosity levels of only 4%-6%.These reservoirs are characterized by their low porosity and low-permeability.Despite the presence of a good source rock in this area,the volcanic debris particles filling the pores,as well as their subsequent devitrification,chloritization,and limonite mineralization,result in pore throat blockage and narrowing.The reservoirs in this area are small in size,exhibit poor reservoir connectivity and lateral continuity,and fail to meet the necessary conditions for commercial-scale hydrocarbon accumulation and migration.

Distribution characteristics and its controlling factor of lacustrine high-quality source rocks in the Bozhong sag,Bohai Bay Basin

Distribution of Paleogene lacustrine high-quality source rocks in the Bozhong sag in Bohai Bay Basin is analyzed through data of geochemistry,geology and well logging,and its differences under the control of climate and tectonics is also well discussed.Distribution characteristics of the high-quality source rocks developed in the saline environment controlled by the climate are quite different from that developed in the rapid subsidence environment controlled by tectonics.The high-quality source rocks in Member 1 of Shahejie Formation developed in the saline environment account for 81.9%of total subsag area,and are distributed widely and extensively.The high-quality source rocks in Member 3 of Shahejie Formation and Member 3 of Dongying Formation is developed in the tectonic subsidence environment,and horizontally,the subsag subsidence rates has a positive correlation with the area proportion of the high-quality source rocks in the sag;vertically,the reduction-oxidation interface of the lake controls the enrichment of highquality source rocks.Controlled by the saline environment and rapid subsidence environment,the highquality source rocks in the Bozhong sag and adjacent areas have three types of development condition:the saline water,the tectonic subsidence,and joint control of the saline water and the tectonic subsidence.The humid climate and low subsidence rate are not favorable for development of high-quality source rocks.