The Study On The Formation And Preservation Of Karstic Reservoir Of Ordovician In Akekule Uplift, Tarim Basin

The Lower-mid Ordovician of the northern part of Tarim Basin is a shallow-marinecarbonate platform deposits which has experienced multi-stage erosions and karstificationproduced karstic reservoir as the mainly reservoir type between the Late Ordovician andLate Devonian , Part of lower-mid Ordovician of the Tahe Oilfield was sub-aerially exposedfor 100 millions of years . During this lengthy exposure period, an extensive karst systemdeveloped that is characterized by an irregular erosional surface, meters-size dissolutioncavities, collapse breccias, sinkholes, paleosols, and fractures. Large caves, corroded fissuresand vugular-solution caves are dominating storage spaces of karstified reservoirs withgreater heterogeneity. It is difficult to study the distribution laws and to prediction them. Theexploration and development of oil and gas in Ordovician have been restricted by thesefactors.In this thesis, an integrated approach was applied that emphasized integration of seismicdata with available conventional core, wireline logs, and geochemical data to analyze theformation mechanism, the controlling factors and the distribution rules of Ordovicianreservoirs were studied by the integrated application of the theories and technologies ofpetroleum geology, tectonic geology, sedimentary petrology, inorganic geochemistry andgeophysics.Five-element model which control the development of karst reservoirs through theanalysis of the structure and dynamic conditions of deposition, burial history and fluidevolution were proposed, the five elements are the structure (basin, tectonic deformation),sequence (structure, style), fluid (surface, ground, deep source), rocks (sedimentary faciesand diagenetic phase) and environmental control. Using the analysis results ofCathodoluminescence and plane- light petrograph, stable carbon and oxygen isotopiccomposition analyses, Sr isotopic composition, trace elements fluid-inclusionhomogenization temperature analyses, as well as Devonian palynological data, Karst geneticmodel is proposed. Ordovician karstification mainly occur in the surface or near-surfacemeteoric conditions, and was affected by the mid-Caledonian and early Hercynian tectonicmovements.The first and secondary episodes of mid-Caledonian karstification were controlled bylithology, exposure time of the erosion and later alteration, thus,the karst of mid-Caledonianperiod is mainly distributed in southern part of Tahe Oilfield where there is the thick upperOrdovician sediments coverage. the principal factor of paleo-karstification duringCaledonian movement was the development degree of early faults and its associated fracture.the main characteristics of paleo-karstification during Caledonian were summarized, that isdistribution controlled by strata, distribution controlled by fault and available reservoir withgreater heterogeneitities. The existence and discovery of Caledonian paleo-karstification hasgreat significance in the process of exploration in peripheral area of Tahe Oil Field.Early Hercynian is the main period of karst formation and preservation of work area.three paleokarst cave horizons were formed due to early Hercynian cycling tectonic uplift,inan order of the lower the newer. Cave distribution is mainly controlled by the lithology,paleogeomorphology, extension fracture and faults, phreatic surface-water discharge karstmode of peninsula type was established.Cave–fractured reservoir with heterogeneities were well developed in Tahe main areawhere the karst development was in the mature stage with high amplitude landscape andabundant geomorphologic elements, including fluvial channels,canyons, fluvial valleys,sinkholes, tower karsts and hills. In YuQi area, late hercynian karstification had moreinfluence on the karstified reservoir, because the superimposition of multi-stageskarstification caused serious strata denudation and deplanation of pimple plain, Large high residual and dissolve ditch were not well developed, in peneplain state and old age of karstdevelopment, karst reservoir are not developed.Tahe Oilfield Ordovician karstification genetic model is proposed on basis ofrecognition of the karst, restoration of the palaeogeomorphology, karst cycles analysis,controlling factors on karst, and the karstification mechanism in this area and its implicationsfor carbonate reservoir are well understood with the use of adjacent interference well testdata ,the inter-well connectivity are analyzed ,then we can distinguish Connected-cavesystems and disconnected caves, which laid a geological foundation for the implementationof different strategies for the production of technology. Multi-phase karsts developmentformed multi-storey cave-style oil storage space with excellent reservoir conditions, andcreated a good oil conduit system and thereby controlled the distribution of the reservoirs.Strontium isotope have better ability of provenance,and have distinct advantages in thedivision of paleokarst stages. Lower⁸⁷Sr/⁸⁶Sr ratios is the character of calcites formed in thekarst of Middle Caledonian, and⁸⁷Sr/⁸⁶Sr ratios of calcites are controlled by the⁸⁷Sr/⁸⁶Sr8r7atios from host rocks. However, the calcites of Early Herlynian paleokarst have higherSr/⁸⁶Sr ratios, and controlled by the terrestrial strontium and the dissolved strontium fromthe wall rocks. Furthermore, the north areas have higher⁸⁷Sr/⁸⁶Sr ratios than the areascovered with O3s Formation, the increased proportion of strontium isotope from host rocks isthe main reason.The interaction between the surface karst-drainage system and the shallow-subsurfacecave-passage system is evidenced by the observation that surface canyons appear to initiatein areas associated with intense sinkhole development. Surface river valleys tend tocorrespond to dip-oriented surface depressions partly related to near-surface cave collapse.During burial into the deeper subsurface, the combination of intrastratal collapse (karstifiedstrata) and suprastratal collapse (postkarst-deposited strata) created large damage zoneshundreds of meters thick and kilometers wide. Coalescedcollapsed paleocave systems can beinterpreted from the unique circular pattern of faults (observed in map view) that areassociated with seismic bright spots. On the basis of static karstification model,theconnectivity of cave system was identified by integration with pressure production and fluidsdata,Beaded reflections on the seismic profile represent high quality reservoir, and theattribute of amplitude variation rate can well depict the distribution of karstified reservoir inareas covered with Upper Ordovician. well log-seismic joint inversion shows that karsticreservoirs were well developed at the 0 to 200m interval beneath the unconformity withcharacteristics of low impedance and lower speed, high porosity.