A discussion on the origin of shale reservoir inter-laminar fractures in the Shahejie Formation of Paleogene, Dongying depression

This paper was conducted on the shale reservoir in the Dongying depression in the Shahejie Formation of Paleogene. Based on detailed core description, rock thin sections, argon ion polishing, scanning electron microscopy (SEM) analysis, CT scanning and carbon-oxygen isotopic test, the formation mechanism of the inter-laminar fractures (or seams) and their development conditions were discussed by comprehensive analysis of the diagenetic evolution features and the reservoir space evolution characteristics. The results show that the laminated composite pattern in Dongying depression was dominated by vertical distribution of laminated calcite and clay minerals. Contrasted to the traditional understanding, development degree of inter-laminar fractures was much lower. Pure fluid between layers cannot exist without framework minerals, and the pre-existing well connected pore or horizontal overpressure micro seam was favorable paths of fluid migration. From the points of inter-laminar fractures origin mechanism and its evolution, the so called seams, were much more complex than traditional understanding that hydrocarbon generation raised the pore pressure and led to the parallel microfractures. But the inter-laminar fractures had little relationship with that whether the lamination was developed or bedding boundary was clear or not. In fact, the shale reservoir inter-laminar fractures were not rigorous fracture. They were controlled by pre-existing inter-granular pore (or seam) or dissolved pore, which distributed along carbonate laminae boundary and were connected by later super pressure, dissolution and the seepage force. And the development conditions were burial depth (bigger than 3 000 m) and the Ro value (>0.6%). Discussion on its origin and evolution has important petroleum geological significance on optimizing reservoir segment and favorable target areas selection.