Intensive research has been done on mudstones (shales) in recent years; however, little research has been done on the fine-grained carbonate rocks. The Shulu Sag located in the south-western corner of the Jizhong Depression, (central Hebei Province) filled by 400–1000 m of Tertiary lacustrine carbonates with abundant accumulation of organic matter is a very important oil producing basin in China. On the basis of the core, thin section and FE-SEM analyses, combined with X-ray diffraction analysis, the dominant minerals present in the Shulu Sag rocks are carbonates (including calcite and dolomite) with an average of 61.5 wt.%. The predominant size of the carbonate is of clay to silt. Most of fine-grained carbonate is dark-colored and composed of regular to irregular, continuous to discontinuous laminae, And some samples are structureless, showing no laminae. Pores in the Shulu calcilutite are of various types and sizes. The pores in the Shulu calcilutite were divided into three types, namely, organic-matter pores, pores within organic matter, interparticle pores, and carbonate dissolution pores. Organic matter and dolomite content are the main factors that control the reservoir quality of the Shulu calcilutite.
The nature and distribution of pores that contain tight oil is a fundamental factor for understanding unconventional petroleum reservoirs. In order to better understand of unconventional petroleum reservoirs, intense research has been done on fine-grained rocks in recent years, particularly those containing abundant organic carbon (Schieber et al., 2007; Aplin and Macquaker, 2011; Loucks et al., 2007, 2009, 2012). A variety of pore types in mudrocks have been identified recently (e.g., Curtis et al., 2012; Fishman et al., 2012; Louckes et al., 2009, 2012; Slatt and O'Brien, 2011;). However, most of the works are focus on clay-rich mudstone or shale and the researches on tight carbonate reservoirs are mainly about their fracture characteristics (e.g. Zeng et al., 2012). Pore types in organic-carbon rich fine-grained carbonate have not been well documented.
There are numerous unconventional resource plays that have potential in China such as in the Songliao Basin, Bohai Bay Basin, and the Ordos Basin. Tight oil is poised to become an important new source of China oil supply. With the increasing difficulty of exploitation of conventional petroleum in these Basins, the unconventional oil resource will be an important supplementary source. Lacustrine carbonate deposits are widely reported in the Tertiary lake basins of the Bohai Bay area, in which they are mostly of autochthonous origin, and are generally considered to be potential oil exploration targets in these basins.
Shulu Sag, the area that is the focus of this study, is located in the south-western corner of Jizhong (central Habei) Depression, which is filled by thick Tertiary lacustrine carbonates with high organic matter contents; they are presently very important oil producing basins in China (Jiang et al., 2007). Oil-bearing tight calcilutite reservoirs of lacustrine origin are found in abundance in the Shulu Sag.