Characteristics And Formation Mechanisms Of Pyroclastic Reservoir

It shows good indication of oil and gas in the Pyroclastic rock reservoir of Yingzuilazi formation of Lower Cretaceousn in many wells of Sankeyushu depression.It also means the study area has great exploration potential.There are still some problems to be solved which include the rock type and the high-quality reservoirs.This paper choose some well core samples in our target regions as the research objects.Experimental techniques we have taken include core observation,thin section identification,Sem,X-ray diffraction,Porosity and permeability,High pressure mercury injection,rate-controlled mercury penetration and nuclear magnetic resonance T2,by which we have studied the reservoir characteristics and formation mechanisms.The results show that Yingzuilazi formation of Sankeyushu depression is Fan-delta sedimentary facies and Lacustrine facies.The sedimentary microfacies are Subaqueous distributary channel,inter-channel,mouth bar and so on.The reservoir of Yingzuilazi formation is mainly composed of clastic rocks(tuffaceous),in addition to sedimentary pyroclastic rocks and pyroclastic lava.The main mineral composition is feldspar and quartz,and also contains a large number of clay minerals such as illite mixed layer.The reservoir is mainly characterized by poor property and mainly from very low porosity to low porosity and ultra low permeability.The reservoir in this area can be divided into three types by physical property and other factors.The average pore throat radius of type I reservoir is the largest,the capillary pressure curve is flat,the pore throat connectivity is good,and the separation is good;The average pore throat radius of type II reservoir is medium,the platform section of capillary pressure curve is short,the pore throat connectivity is good,and the sorting is medium;The average pore throat radius of type III reservoir is the smallest,the capillary pressure curve is steeper,the pore throat connectivity is poor,and the sorting is poor.Although the available pore sizes of the three types of reservoirs are similar,but the number of pore throats varies greatly.The number of pore throats in type I reservoir is more,while the number of pore throats in type III reservoir is less.Further statistical analysis shows that the dominant sedimentary microfacies are the basis for the formation of high-quality reservoirs.The physical properties of pyroclastic reservoir in Yingzuilazi formation are controlled by sedimentation.Among them,the physical property of Subaqueous distributary channel sedimentary reservoir is the best and the followed is mouth bar.The other microfacies almost do not develope high-quality reservoirs.Constructive diagenesis is another key factor for the formation of high quality reservoirs.The study area is in the middle diagenetic member A.It has experienced multiple diagenesis including fusion,compaction,dissolution and cementation.Dissolution is the main factor controlling the reservoir in this area.Organic acids formed during the maturation and evolution of regional high-quality source rocks are likely to be the main sources of dissolution fluids.In addition,fractures caused by tectonism can promote the formation of effective pore network,which is also an important factor affecting the formation of reservoirs in this area.