Study On The Formation Periods And Filling Characteristics Of Carboniferous Volcanic Fractures In Chunfeng Oilfield

Chunfeng Oilfield is located in the northeastern of Chepaizi area,which is belongs to the thrust fault zones in the northwestern margin of Junggar Basin.Under the multi-phase compressional thrust structure setting,a large amount of fractures associated with fault were formed within the Carboniferous volcanic rock.The exploration practice indicated that the inside fault system of Carboniferous is complex and result in the multistage forming,multistage reformation and multistage filling characteristics of natural fractures.Because of the lack of systematic understanding of formation periods and filling characteristics of fractures,the influence of fractures on hydrocarbon accumulation is not clear.Aiming at these problems,the core,thin sections,lab analysis,image logging and seismic data are comprehensively utilized to analysis the fault characteristic and fault periods,furthermore,the types and characteristics,the formation period,the filling characteristics of volcanic fractures,the quantitative evaluation of fault symbiotic fracture and fracture effectiveness are studied with the combination of geological structure analysis and lab testing methods.The main results and understandings are illustrated as follows:(1)The fault grade and development period of Carboniferous volcanic rock are classified.Study revealed that the fault types mainly are pressure thrust fault,and can be divided into grade ?~?,the fault periods are divided by its times into middle Hercynian period,late Hercynian period,Indosinian-Yanshan period and Himalayan period.The various stage of structural movement controls the types,scales and orientation distribution of different periods of fractures.(2)The formation periods of Carboniferous volcanic fractures is revealed.One period diagenetic fracture and three periods structural fracture can be identified by cutting method.By means of experiments and tests,such as fluid inclusion,acoustic emission experiment of carbon-oxygen stable isotopic analysis of fracture fillings,inverted the genetic sequence and rocks breaking stages of fractures,the volcanic fractures have experienced three fluid activities and the the fractures of Carboniferous reservoir were formed in four periods.Hercynian-Indosinian stage is main forming period of fractures of volcanic reservoirs because of the most actively tectonic movement.Further more,the imaging logging analysis can confirm the formation sequence of different fractures into NE or EW,NW,NNW and NE.(3)The filling characteristics of Carboniferous volcanic fractures is revealed.The fracture filling degree can be divided into non-filling,semi-filling and full-filling and it relate to occurrence,depth,unconformity surface and tectonic effects.Research suggests that the high angle oblique crossing fracture and vertical fracture have a large proportion in the non filling and have a good effectiveness of fracture.The fractures in leached layer and hydrolysis layer of unconformity structure have a lowest filling degree.The fractures near faults have higher filling contents,in addition,with the increase of distance to the fault,the proportion of fullfilling fractures gradually decreased while the non-filling fractures increased greatly.(4)Established the fault-controlled fracture density function,the effective control distance is 5.5km,3.0km and 0.9km for?~? fault respectively.A fault strength analysis method is proposed to to transform the regional fault information dimension into fracture information dimension,which achieves the detailed description and quantitative characterization of faultrelated fracture development of Carboniferous.The fractures are mainly distributed at banded or patchy along?and ?fault.So the interchanges,turning place and end place are all high fracture density area.(5)The fracture validity of Carboniferous volcanic fractures can be divided into 3 grades.Evaluation result shows that when the fracture density is high and the orientation of the fracture is consistent with direction of the maximum main stress,the fractures open largely and are connected with each other.At this point,the fracture validity of this region is good and also have a higher richness of oil and gas.