Development Characteristics Of Reef-shoal Reservoir And Conductivity Of Shoal In Typical Outcrops,the Northwest Tarim Basin

High-quality reservoir phase belts prediction of carbonate reef-shoal is the focus that petroleum explorationists have always been concerning.Fine structure anlysis and reservoir property study of outcrop depositional system is the key to clearing the distribution of the reservoirs and summarizing the favorable exploration area.In the Akesu outcrop area of the Tarim Basin,the structure of the reef-shoal depositional system and reservoir property in the lower Cambrian group Xiaoerbulake formation and Middle Ordovician Yijianfang fomation are analyzed through detailed field investigation,lateral tracing and contrast of sedimentary bodies,systematic sampling and indoor test analysis.We conducted several groups conductivity experiments of the tight carbonate intra-platform shoal reservoirs,which is traditionally considered as low porosity and low permeability,and analyzed the applicability of stimulated reservoir volume in this kind of reservoir.It is considered that the reef-shoal developed in Cambrian is large.The reefs show different stacking pattern influenced by relative sea level.The reef-shoal developed in Ordovician is small,but the distribution range of the shoal is larger,which often has lateral migration influenced by the palaeoggeomorphology and the sea level.Platform structure,micro palaeogeomorphology and sea level jointly constraining the development of microbial reefs,and the development of reefs reacts on the development of platform structure and topography.The lower Cambrian reefs are mainly composed of the stromatolite and thrombolites crystal dolomite,and the shoal is mainly composed of residual grain crystal dolomite and grain dolomite,and the Middle Ordovician reefs are composed of bafflestone,mainly including basbass and sponges,and the surrounding pellet beach is spine limestone.The clastic limestone consists mainly of crinoideas,shellfishes,and gastropods.The reservoir spaces are mainly composed of inter-crystalline pores,inter-grain pores and fractures,and are not very developed.They belong to very low porosity and low permeability reservoirs.The application of stimulated reservoir volume is an innovative attempt for the tight intra-platform shoal reservoirs with undeveloped matrix holes.In the fractured experiment,it is found that there are two kinds of fractures in the reservoir,one is the self-propped opened by the fracture slip and the other is proppant-propped which is opened by proppant to hold the fracture.Under low closure pressure,self-propped fractures can provide significant conductivity,which is better than that of flat proppant fractures.With the increase of closing pressure,the fracture conductivity of proppant gradually overtake the self-propped fractures,and the self-propped fracture conductivity dramatically decreases,and proppant fractures decrease steadily.After the experiment,the samples cracked to varying degrees and enhanced the permeability of the reservoir as a whole.In addition,the distribution of asperities and the degree of roughness(the degree of engagement of fractures)affect the initial conductivity of fractures.When the formation pressure is large,a proper increase of the proppant concentration can overcome the unpredictable fomation properties such as the self-propped fractures roughness and keep the stability of the conductivity.