Characteristics of pore systems in the oil-bearing sandstones of the Dongying Depression, Bohai Bay Basin

Abstract

The Dongying Depression is an oil-bearing rift depression and mainly consists of beach bar, turbidite, and sublacustrine fan sandstone reservoirs. The pore systems of these sandstones have a crucial impact on maintaining and enhancing oil recovery, but the complexity of pore-throat structure and distribution cannot be described in detail using a single method; in particular, the pore systems of tight sandstones are principally composed of small-scale and discontinuous pores. Petrological observation of these sandstones demonstrated that the main pore systems in beach bar and sublacustrine fan sandstones are composed of micropores related to clay minerals, and few residual intergranular and secondary intragranular pores, because of intense compaction and cementation. In contrast, turbidite sandstones preserve abundant intergranular and secondary intragranular pores due to moderate compaction. High-pressure mercury injection (HPMI), nuclear magnetic resonance (NMR), and X-ray computed tomography (X-ray CT) were conducted to explore the features of full-scale pore systems. The NMR and HPMI demonstrated that turbidite sandstones have larger pore and throat sizes than beach bar and sublacustrine fan sandstones, consistent with the petrophysical characteristics and petrological observations. Moreover, X-ray CT analysis also indicated that turbidite sandstone has a larger pore radius and a higher proportion of connected pores, than beach bar sandstone.The fractal characteristics of pore systems are quantificationally described by NMR and HPMI. Fractal dimension (Df2) of pore sizes (T2 > 1 ms) show a positive relationship with BVI value, as well as negative relationships with T2peak and T2gm; and fractal dimension (DfS) from HPMI exhibit a positive correlation with the radius of small pore throats (<rapex), indicating that the high proportion of small pore structure increases the heterogeneity of sandstone reservoirs. The fractal dimension of tortuosity (Dτ) is negatively correlated with permeability and can be regarded as parameters to quantificationally describe reservoir quality. Therefore, the simplified numbers of fractal theory obtained from NMR and HPMI experiments provide insights and conveniences for evaluating and comparing pore systems in sandstone reservoirs in the Dongying Depression.