Experimental study on reservoir characteristics and oil-bearing properties of Chang 7 lacustrine oil shale in Yan’an area, China

Abstract

The Chang 7 Member shale of the Upper Triassic Yanchang formation in the Ordos basin is a hot spot in petroleum geology research. In this study, considering the Chang 7 shale in the Yan’an area as an example, the full-scale pore size characterization of lacustrine shale was realized based on the scanning electron microscopy image gray correction method, nitrogen and carbon dioxide adsorptions, and high-pressure mercury intrusion tests. In addition, the pore structures and oil-bearing properties of the Chang 7 shale were systematically studied. The results show that the Chang 7 shale is rich in organic matter, with an average total organic carbon value of 4.69% and an average R 0 value of 0.9%. It is in the mature-wet gas (crude oil-associated gas) stage. There are certain differences in the development characteristics and pore size distribution of different types of pores in shale. The statistical results showed that the pore diameters of the intergranular pores and intragranular dissolved pores were significantly larger than the intercrystalline pores of clay minerals and the organic pores. The organic pores in solid bitumen are extremely developed, whereas the organic pores in kerogen are relatively underdeveloped. The lower limit of the effective pore size of shale is 20 nm. The network system composed of inorganic pores-microcracks-organic matter-organic pores and siltstone laminae provides important channels and retention spaces for the migration of shale oil and gas within the source. This study found that the proportion of movable oil in sandy layers is relatively high, followed by shale with sandy laminae, whereas pure shale has the lowest proportion of movable oil. Therefore, the degree of sandy laminar development, the abundance of organic matter, and the degree of thermal evolution are the key geological factors that control the porosity and oil-bearing properties of shale oil reservoirs.

Changes in Nanoscale Pore Structures and Mesopore Connectivity as a Result of Artificial Maturation of a Source Rock from the Shanxi Formation, Ordos Basin

“…Therefore, it is generally believed that secondary OM pores are progressively developed as the OM maturity gradually increases. For shale oil reservoirs, the pore-filling of residual bitumen can exhibit an artificial decrease of porosity, while oil washing treatments will show the distribution of residual oil around 40 nm in diameters, which is linked to the threshold for oil movement in shale; , this is overall consistent with the findings of this work with a particular focus on mesopores.…”

“…The pore volume measured by the (U)­SANS method includes all pores, while the pore volume measured by fluid intrusion methods (such as N 2 and CO 2 ) is only the volume of pores accessible from the sample surface. LNA is widely used to characterize the pore structure of both mesopores and macropores, with the pore volume obtained through the BJH model. , In this work, by comparing the data of LNA and (U)­SANS in the range of 2–50 nm, which is the most commonly occurring pore sizes for a residual oil presence as shown before and after solvent extraction in the literature. − The percentage of gas-inaccessible mesopores (of total mesopores) can be calculated by where F inaccessible pores indicates the percentage of inaccessible mesopores in the total mesopores; V USANS is the mesopore volume obtained by (U)­SANS; V BJH is the volume of accessible mesopores obtained by the BJH model.…”

“…Furthermore, pore generation and types in the thermal maturation of OM-rich shale involve a dynamic process of pore-filling and subsequent secondary cracking of residual bitumen along the petroleum expulsion process. ,− , Alcantar-Lopez (2016) demonstrated the progressive changes from subspherical units with increasing maturity to form a cross-linked network of OM for kerogen structures. Such changes, when being associated with oil generation/swelling and subsequent oil expulsion/shrinkage, provide the routes for pore development during maturation .…”