Controlling effect of transtensional fault system on shale oil enrichment of mud shale layer in continental faulted lacustrine basin

This paper is the result of oil and gas exploration engineering.

Objective Under the background of oblique-tension in the Cenozoic, a series of transtensional faults were formed in the Bohai bay basin. Previous studies on the characteristics and reservoir controlling effects of transtensional faults were mostly carried out on the basis of sand oil and gas, but the relationship between transtensional faults and shale oil accumulation was not involved.

Methods Based on the structural characterization of the transtensional faults, the controlling effect of transtensional faults on shale oil enrichment was discussed.

Results There are three types of transtensional fault system in the mud-shale system of Dongying Sag, the main faults controlled by the sag boundary determined the characteristics of strong segmentation of several secondary depressions in the Donging sag. The main faults in the sag can cut or stagger the main fault system, the secondary faults were often associated with the main faults, resulting in the fractured mud shale formation. The combination types of transtensional faults were regular, the seismic profile was dominated by negative flower-like structure, which can be divided into three types: semi-flower-like, tree-flower-like and coiled flower-like. In plane, there were four types, including echelon, horsetail, broom and arc type. The transtensional faults control shale oil accumulation obviously. It controlled the distribution of shale structural zones, and the deep-water sedimentary environment was conducive to the extensive development of shale system. A variety of tectonic style fracture belts were formed, with the dual structure of "fault damage zone and fault core", the high-density fracture zone inside the fault damage zone can significantly improve the reservoir performance of shale oil reservoir and constituted the key element of shale fracture sweet spot. Based on the gradient structure tensor attribute and attribute proportion fusion technology, the (semi-) quantitative prediction of the distribution range of fracture damage zone was realized. Shale reservoir formation included three stages: shale hydrocarbon generation pressurization, tension-torsion tectonic stress activity adjustment, and shale oil accumulation.

Conclusions It has been found that shale oil reservoirs are mostly distributed in the areas where transtensional tectonic stress is concentrated released, such as transtensional tectonic fracture belt, shale formation uplift belt and tilted deformation belt. The shale fracture zones in these regions are well developed, forming an open and circulating fracture network system in space, and shale oil is the most abundant.