| The Jurassic Da'anzhai Member of the Middle Sichuan Basin is a typical tight oil reservoir with a porosity of less than 2%and a permeability of less than 0.1×10-3?m2.As a whole,this member has a low exploitation level,a huge development potential,and good crude oil properties with the generally oil-bearing of the reservoirs,1/3 of the wells have natural production capacity.In the context of tight oil reservoirs,the evaluation of original fractures and reservoir volume fracturing are urgent requirements for obtaining production capacity.In the reservoirs of Da'anzhai Member,macro-fractures are generally developed,in the traditional identification of the cracks in this section,the evaluation of fractures did not form a system,and the control factors of the crack distribution were unclear.It is urgent to evaluate the fractures systematically for the guiding of selection of the crack-rich areas.At the same time,for the reservoir fracturing technology which used in the Da'anzhai Member,it is also necessary to clarify the characteristics of the ground stress and the brittleness distribution characteristics of the member.Based on the research of previous studies,this paper evaluates the fractures in the Da'anzhai Member from the aspects of fracture characteristics,fracture classification and fracture distribution,besides,the stress and brittleness of rocks were analyzed in this paper,in order to provide technical support for the development of favorable zone optimization in Da'anzhai Member.In this paper,through the research methods of cores,thin sections,cast thin sections,and electron microscopy,the fractures in the Da'anzhai Member are described,and the development characteristics of fractures are analyzed.It is found that the composition of the rock affects the characteristics of the fracture:the shell grains control the cracks between the shells and in the shell,the calcite grains control the intergranular joints,which were easily broken along the weak intergranular surface under the action of tectonic stress.According to the analysis of fracture characteristics,cracks were classified according to their causes and scales,and the cracks were identified under the condition of non crack influence factors and gradual constraints.The identification results show that:(1)Large-scale cracks were reduced in the resistivity curve like tooth or fingers,the resistivity was lower than 3000?·m,and the small-scale cracks often have a "platform gap" in the resistivity curve,the micro-scale fractures are characterized by high amplitudes in resistivity with a finger-type;(2)the high-angle cracks and oblique cracks are not obvious in the log curves,low-angle and horizontal cracks shows a increase in sound waves,a decrease in resistivity like fingers;(3)no matter what kind of the scale of the cracks,with the increase of the degree of development,there would be a trend of decreasing in the resistivity curve.According to the crack recognition results and theoretical formulas,the model of the fracture size,origin,development degree identification model and fracture parameter interpretation model were established.The self-written crack interpretation program was used to explain the fracture of Da'anzhai Member in Zhongtai Mountain area,and the distribution characteristics of the fracture favorable areas were analyzed,it was found that the high developmental fractures are mainly distributed in the Dal and Da13 sub-member,and are locally formed into patches,in the plane,it was mainly distributed near the well N14 and S2,the distribution of brittle favorable area is similar to that of high crack development area,which was concentrated in the central and northeastern regions with the overall brittleness index between 45%and 60%,in addition,the distribution of crack was also controlled by the structure.And the further using ant tracking to predict the seismic body cracks in the G101 area showed that the overall distribution of fractures was in the southeast-northwest direction,and partially in the northeast-southwest direction.This paper provides a systematic research plan for the evaluation of fractures of Da'anzhai Member by using conventional logging,and provides a more detailed log interpretation reference for the favorable area. |
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