Reservoir characteristics and factors influencing shahejie marl in the shulu sag, bohai bay basin, eastern China

Abstract

Shahejie marl in the Shulu Sag is a crucial resource for unconventional hydrocarbon exploration in China. Although breakthroughs have been made in tight oil exploration in this area, the mechanisms underlying the formation of this marl reservoir and factors controlling its ‘sweet spots’ have not been thoroughly studied. To understand the pore structure characteristics and factors influencing the marl reservoir, we analyzed core samples from Wells ST1 and ST3. A series of experiments was conducted on the samples, such as X-ray diffraction, focused ion beam scanning electron microscopy, micro-CT, and total organic carbon test. Additionally, the physical properties of different marl rock fabrics were studied with auxiliary tests, such as mercury intrusion capillary pressure analyses, nuclear magnetic resonance, porosity and permeability tests, and thin-section observation. The results revealed that the marl reservoir is characterized by low porosity (1.61%) and low permeability (2.56mD). The porosity and permeability (1.61% and 3.26mD) of laminated marl were better than those (0.92% and 1.68mD) of massive marl. Clay minerals and quartz content in laminated (11.8 and 8.2%) was less than in massive marl (16.2 and 13.3%). The marl pores include intercrystalline pores, dissolution pores, and microfractures. Additionally, the laminated marl pores were primarily distributed along the dark lamina, with good connectivity. A few isolated and uniform holes were observed in the massive marl. Influenced by rock fabric and mineral composition, layered fractures were mainly developed in the laminated marl, while structural fractures were the main type of microfractures in the massive marl. The primary sedimentary mechanism was the main geological action underlying the differences in marl rock fabric; this mechanism affects the physical properties of the marl reservoir, which are key factors to be considered when searching for the marl reservoir ‘sweet spots’. Particular attention should be paid to these factors during tight oil exploration and development in similar sedimentary basins.

Reservoir Characteristics and Development Model of Subaqueous Pyroclastic Rocks in a Continental Lacustrine Basin: A Case Study of the Chaganhua Subsag in the Changling Fault Depression, Songliao Basin

“…The Lower Cretaceous Huoshiling Formation (K1h), Shahezi Formation (K1sh), and Yingcheng Formation (K1yc) were mainly developed during the faulting period of the Changling fault depression, which is characterized by sedimentary filling, simultaneous superposition of multiple eruptions of volcanic rocks, and late modification [46] (Figure 2). The study area is the Chaganhua subsag, which is located in the eastern gentle slope zone of the Changling fault depression, with a near north-south strike, and the Huoshiling Formation, Shahezi Formation, and Yingcheng Formation in the area basically distributed along its western depression-controlling fracture [48]. The Chaganhua subsag has undergone a transformation from fault to depression, with the Huoshiling stage being the initial tensional faulting stage and the basement fractures being more developed (Figure 1d).…”

“…The Chaganhua subsag has undergone a transformation from fault to depression, with the Huoshiling stage being the initial tensional faulting stage and the basement fractures being more developed (Figure 1d). The deposition process was accompanied by strong activity of controlling trap faults, which formed massive volcanic rocks; at the end of the deposition of the Huoshiling Formation, the study area was extensively uplifted and denuded, forming regional The study area is the Chaganhua subsag, which is located in the eastern gentle slope zone of the Changling fault depression, with a near north-south strike, and the Huoshiling Formation, Shahezi Formation, and Yingcheng Formation in the area basically distributed along its western depression-controlling fracture [48]. The Chaganhua subsag has undergone a transformation from fault to depression, with the Huoshiling stage being the initial tensional faulting stage and the basement fractures being more developed (Figure 1d).…”