Characteristics And Formation Mechanisms Of Continental Mixosedimentite Reservoirs

As a transitional and special type of sedimentary rock between clastic and carbonate rocks,the mixed siliciclastic-carbonate rock(mixosedimentite)has been investigated with focus on description,categorization,terminology and basic sedimentary features.In addition,some works have studied their petroleum geological.significance preliminarily.Thus,the study of the petroleum geological significance of the mixosedimentite is a frontier and cutting-edge field of sedimentology and petroleum geology.Due to a transitional deposition,such works are mainly reservoir studies.Thus,to address this scientific issue,we conducted a case study in the northwestern Qaidam Basin,which is a typical superimposed and multicyclic basin in West China.Considering the current exploration focus,the moderately to deeply buried Paleogene to Neogene was studied.We mainly report basic reservoir characteristics and discussed reservoir formation mechanism.Thus,the results may provide complement to basic theoretical research on reservoir geology,and may also provide valuable information for the regional petroleum exploration.Firstly,we analyze basic petrological features of the mixosedimentites,revealing the criteria for identifying the rocks.It is found that the development of mixosedimentite has macro sedimentary setting and micro petrological features,and is controlled by sedimentary facies.This is the fundamental for studying the mixosedimentite reservoir.A total of 286 core samples from 32 exploration wells were comprehensively studied by careful observation of drill cores and hand specimens and systematic thin-section microobservation.It is showed that typical continental mixed rock system is developed in the study area,including clastic,carbonate and mixosedimentite rocks.Sedimentary facies plays a control in these lithologic features.Fluvial and delta facies are beneficial to the development of clastic rocks,deep,mid-deep and shallow lake facies are beneficial to the development of carbonate rocks,and the lateral and/or vertical overlapping positions of fluvial and lacustrine environments are favorable for the development of mixosedimentites.Clastic rocks are well developed in different structural belts of the study area;in contrast,the carbonate rocks are not so well developed.As to the mixosedimentites,their occurrence varies between the belts,typically found in the Nanyishan-Dafengshan and Youquanzi-Katmerica belts.On the above basis of petrological features,another two lines of basic reservoir characteristics were studied,including reservoir space type and physical property.The results showed that the reservoir space can be divided into three end-member genetic types:nearly disappeared primary pores,secondary dissolution pores and fractures with multiple origins.These three types constitute two main combinations:residual primary porosity + secondary dissolution pore + fracture and secondary dissolution pore + fracture.The reservoir physical property is generally characterized by low porosity and extremely low permeability,and decreases with increasing burial depth.However,relatively high-quality reservoir is still developed,mainly nearby the depth of 2500 m and 4200 m,characterized by dissolution pores and fractures in reservoir space.The above represents a relatively comprehensive description of reservoir basic characteristics.We further discuss the evolution of the reservoir,i.e.,diagenesis.It is showed that compaction and cementation belong to the type of destructive diagenesises,being unfavorable for the development of reservoir.They are the cause of reservoir densification.However,some cementation,dominantly of calcium and gypsum,may be termed as retention diagenesis.This is due to two points.First,the cementation increases the resistance and brittleness of reservoir.Second,they can be eroded by hydrocarbon-bearing acidic fluid and thus to form favorable reservoir.By comparison,dissolution and fracturing are beneficial to reservoir development in general,especially for the mixosedimentite such as marl(pelitic dolostone),grey(politic)siltstone.This it is because such rocks have resistance feature which is beneficial to the development of fractures,and grey and gypsum components of these rocks are also beneficial to dissolution.Thus,it is showed that the petrology has an ultimate control on the diagenesis type of fracturing and dissolution.In addition,as the conduit of fluid migration,the fracture provides favorable conditions for the widespread occurrence of dissolution,leading to the increase of permeability.Thus,the dissolution is a key to the reservoir formation.Integrated the above results,the origin of the reservoir can be summarized into "three factors",i.e.,lithology is the foundation,fracture is the condition,and dissolution is the key.The unique petrological composition of mixosedimentite provides a basis for the development of dissolutions and fractures,which are the main reservoir space types of relatively high-quality reservoir.Lithological composition of the mixosedimentite is complex and generally contains brittle minerals,exhibiting rigid features.Therefore,fractures are commonly developed,and a large number of calcareous and gypsum components provides material base for dissolution.A general reservoir formation model for continental(lacustrine)mixosedimentite high-quality reservoir was established,based on a case study in the Nanyishan area,which is the only proved case of hydrocarbon accumulation in the study area.Then,we further analyze the key control on reservoir development,i.e.,dissolution,aiming at a better understanding.It is indicated that the dissolution here belongs to burial dissolution,and the fratures caused by faults are the main conduits for the migration of the dissolution fluid.The trace element analysis on diagenetic calcite showed that Fe,Mg and Mn can be used as a good tracer to decide the origin of the dissolution fluid.Mn concentration in the diagenetic calcite is specifically indicative of organic-rich fluid.Based on this criteria,combined with the compositional characteristics of trace elements of authigenic minerals,hydrocarbon burial-thermal evolution history,and hydrocarbon generation history,we proposed that the hydrocarbon-bearing acidic fluid(organic acid)is the main body of the reservoir dissolution fluid.Finally,integrated all the above results,in combination with the geological background,we suggest favorable reservoir for exploration,vertically including E3-N,with the key depth nearby 2500 m and 4200 m and spatially including three areas(e.g.,Xianshuiquan-Honggouzi-Youquanzi-Katmerica area(Ni),Xianshuiquan-Youquanzi-Nanyishan area(E3)and Jiandingshan area(E3-N1)).