Characteristics and tightening process of reservoirs in the Paleogene Kongdian Formation, Zaoyuan oilfield, Cangdong Sag, Bohai Bay Basin

The Paleogene Kongdian Formation in the Zaoyuan oilfield within the Cangdong Sag of the Bohai Bay Basin, a faulted lacustrine basin, contains tight-sand reservoirs. However, there remains a lack of consensus on the relationship between the reservoir tightening and hydrocarbon charging. In this study, we examine representative samples from the 2 nd member of the Kongdian Formation (also referred to as the Kong 2 Member) in the Zaoyuan oilfield. By integrating thin section observations, field emission scanning electron microscopy (FE-SEM), high-pressure mercury injection (HPMI), low-temperature nitrogen adsorption, and fluid inclusion analysis, we reconstruct the reservoir tightening process and analyze its impact on hydrocarbon accumulation. The results reveal the presence of three types of microfacies in the Kong 2 Member: channel, lobe, and levee. Reservoirs in this member consist primarily of finegrained sandstones and siltstones, with pores dominated by primary intergranular pores and secondary dissolution pores. The reservoir tightening in the Kong 2 Member is governed by sedimentary components and textural maturity, along with the imprint of diagenesis. The tightening process can be divided into three stages: moderate compaction-induced porosity reduction, weak to moderate dissolution-induced porosity increase, and weak to moderate cementation-induced porosity reduction. Reservoirs of varying sedimentary microfacies show differential reservoir tightening characteristics. Compared to sandstone reservoirs of the lobe microfacies, those of the levee and channel microfacies exhibit higher degrees of cementation-induced tightening due to a more open diagenetic chemical reaction system. Overall, in the Kong 2 Member, reservoir tightening occurred earlier than hydrocarbon accumulation. Variation in the tightening intensity across different sedimentary microfacies of sandstones is identified as the primary cause of the differential hydrocarbon enrichment. Additionally, natrolite cementation formed after hydrocarbon charging can be observed locally.