The Study On Fracture Development Rule Of The Typical Buried-hills In The Jizhong Depression

Fractures are the main storage spaces and migration channels for fluid flow in buried-hill reservoirs,which affected the enrichment and production of oil and gas.Based on data of cores,thin sections,drilling,imaging logs and conventional logs,the fracture origin type,distribution characteristics and development rule in the Neoproterozoic-Late Paleozoic carbonate reservoirs of the typical buried-hills,i.e.Hexiwu,Wenan,Niudong,Changyangdian,Suning and Sunhu,in the Jizhong Depression are studied in the paper.There developed four types of fractures,tectonic fracture,diagenetic fracture,dissolution fracture and weathered fracture,in carbonate buried-hills in the Jizhong Dpression.Among them,tectonic fracture with high dip-angle is the main type.The strikes of natural fracture are NE-SW,E-W and NW-SE orientations.The fractures have good effectiveness influenced by fault,in-situ stress and dissolution.The development degree of fracture is controlled by lithology,bed thickness and fault,which lithology is the best.In general,dolomite is more favorable for fracture development than limestone.Along with clay content adding,the development degree of fracture decreases obviously.The development degree of fracture in the buried-hills of Niudong,Changyangdian,Suning and Suhu which dolomites of the Wumishan and Gaoyuzhuang Formations are the main reservoirs are more developed than that in the buried-hills of Hexiwu and Wenan which the Ordovician limestones are the main reservoirs.Using finite element numerical simulation method based on the palaeotectonic stress field of fracture formation,the fracture development models in two types of buried-hills are established.In the hammock buried-hill,fractures are mainly developed in the top of the hill and controlled by lithology in the deep,which fractures are more developed in dolomites than that in limestones.In the fault block buried-hill,fractures are controlled by lithology,bed thicknes and fault,which the development of fracture is stronger nearby fault than that far from the fault site.