Diagenesis Types of Fine-grained Sedimentary Rocks in the Ek2 of Cangdong sag and Their Influence on Reservoirs

Cangdong sag is an important oil-rich sag in Bohai Bay Basin, eastern China. Organic-rich fine-grained sedimentary rocks with a thickness of 400m are developed in the second member of Paleogene Kongdian Formation, which have good shale oil resource potential. However, there are few studies on diagenesis of fine-grained sedimentary rocks in Kong-2 member of Cangdong sag, especially the study on pore-increasing and pore-reducing mechanism based on diagenesis is insufficient, which restricts the fine evaluation of shale oil reservoir performance in different lithofacies and stratigraphic units.

Based on the qualitative analysis of core observation description, rock thin section analysis and scanning electron microscopy of Well G108-8 , Well GD12, and Well GX12X1 in Cangdong sag, combined with quantitative characterization techniques such as XRD whole rock analysis, XRD clay mineral analysis and energy spectrum analysis, it is clear that the diagenesis of fine-grained sedimentary rocks in the second member of Kong in Cangdong sag includes compaction, cementation, dissolution, metasomatism, clay mineral transformation and thermal evolution of organic matter. Because of the widespread development of cristobalite in Cangdong sag, a large number of cristobalite cements are filled in cracks or intergranular pores. The thermal evolution of organic matter is mainly characterized by internal hydrocarbon generation pores and marginal contraction joints. Under the scanning electron microscope, it is observed that the organic matter in the second member of Kong Formation in Cangdong sag is easy to combine with clay minerals to form 'organic matter-clay aggregate'. The conversion process of clay minerals can catalyze the hydrocarbon generation of organic matter, thus promoting the development of organic pores in the aggregate. Corrosion is common in the second member of Kong in Cangdong sag, mainly developed feldspar and carbonate rock corrosion. Under the influence of organic acids released from hydrocarbon generation of organic matter, corrosion pores formed by dissolution are more likely to be seen in unstable minerals in carbonate or clastic lamina adjacent to organic matter.

According to the data of porosity and permeability, pressure porosity and permeability, gas adsorption, high pressure mercury injection and nuclear magnetic resonance, the reservoir characteristics of Kong 2 member in Cangdong sag are analyzed. The results show that the porosity of the fine-grained sedimentary rock reservoir in Kong-2 member of Cangdong sag is between 0.24% and 9.35%, with an average of 2.95%. The permeability is between (0.01-25.3) × 10-3μm2, with an average of 0.613 × 10-3μm2, and the overall reservoir is ‘ultra-low porosity and ultra-low permeability’. Pores include inorganic pores, organic pores and micro cracks. The pore size is small, mainly micro-nano pores.

The influence of diagenesis on the reservoir of the second member of Kong Formation in Cangdong sag is complex, including the mechanism of increasing porosity and reducing porosity. The pore-increasing mechanism includes thermal evolution of organic matter, dissolution and clay mineral transformation;compaction and cementation are the main factors affecting the porosity and permeability of the reservoir, which belong to porosity reduction for the reservoir of Kong 2 member in Cangdong sag.