Reservoir Characteristics And Geological Modeling For The Typical Gas Reservoir In Callovian-oxfordian Of Area A In Amu Darya Basin

In this paper, the study area is located in the junction of Uzbekistan and thenortheast of Turkmenistan. The target layer is that from compact limestone (XVI) tothe interbred of anhydrite and limestone (XVac) of Callovian-Oxfordian in the upperJurassic of Mesozoic. According to the assignments and purposes of the paper, theauthor combines with the status of carbonate sequence stratigraphy and sedimentaryfacies in and abroad, utilizes the theory of Vail’s classic sequence stratigraphy,carbonate sequence stratigraphy and sedimentology as the direction, integratesdrilling, logging, core observation thin section analysis and seismic data, and thenclassifies the3rd order sequence,systems tract and sedimentary facies in the studyarea,which are launched and compared. After that the whole area sequencestratigraphy framework is built. Sedimentary facies characteristics of the spread ofVertical and horizontal distribution and plane distribution are analyzed under theisochronous unit of systems tract, then the depositional model is established. Basedon this, taking the theory of petroleum geology and carbonate reservoir geology asthe guidance, integrating analysis and test results, rock and mineral analysis resultsand seismic reservoir inversion results, the author systematically studies thereservoir rock characteristics, reservoir space types, diagenesis and porosityevolution, pore structure, physical property, reservoir type, interbed characteristicsand distribution of reservoir space, and analyzes controlling factor of reservoirdevelopment. Finally, the author comprehensively evaluates the reservoir andpredicts favorable and more favorable reservoir belts. On the basis of the abovestudy, guiding by the theory of development geology and gas reservoir engineering,gas reservoir characteristics in the study area is analyzed; reservoir property model of the typical gas reservoir is established.Amu Darya basin is a Mesozoic basin which develops in the background ofHercynian syncline, which formation and development through three stages that arebase formation, stable subsidence and collision transformation. Thousands meters ofmarine and continental intertwined deposition, which is coal-bearing clastic rockconstruction and main source rock, is deposited in the middle Jurassic. A large set ofcarbonate rock which is the main reservoir is deposited in Callovian–Oxfordian oflate Jurassic. The study area is in a closed marine depositional environment becauseTulan platform started to return to in Kimmeridgian–Tithonian of late Jurassic,anda large set of evaporite rock which is the main cap-rock is built up in the strong-aridclimatic condition.Integrated findings of chrono-stratigraphy, lithostratigraphy, biostratigraphy,seismic stratigraphy, combined with sedimentary facies and tectonicbackground,four sequence boundaries (both in Vail Class Ⅱ sequence boundary)and three maximum flooding surface can be identified in Callovian-Oxfordian inthe study area, which is divided into three third order sequence. Within each of them,transgressive systems tract and high-stand systems tract can be identified, withoutlow-stand systems tract.Through the analysis of drilling wells in study area, especially on the basis ofcore observation and thin section authentication, the author believes that limestones,dolomites and gypsums are the main types of rocks, and recognizes a sort ofsedimentary facies such as evaporative platform facies, restricted platform facies,open platform facies, platform edge facies and platform foreslope facies in studyarea. What’s more, through dividing sedimentary subfacies and microfacies,analyzing horizontal distributing character of sedimentary facies within sequenceframework, establishing sedimentation model, and analyzing main controlling factorof sedimentary facies, the author makes such a conclusion that the development ofCallovian-Oxfordian sedimentary facies in study area is mainly influenced bytectonic movements, sea level changing, climate and ancient landform, amongwhich sea level changing performs is the most influential factor.According to the sedimentary environment, the type of reservoir rocks can bedivided into two types, which are biohermal limestone and organic bank limestone.The type of pore spaces, which includes pores, limestone caves and fractures, isrelatively simple. They have mainly undergone compaction, pressolution,cementation, dissolution, packing action, rejuvenation of crystals, cataclasis and dolomitization, among which dissolution has the most significance. The porosityevolution has undergone synchronous period-early diagenesis stage, early period ofmesozone rock stage and late period of mesozone rock stage. Big, mediumporosity-coarse, medium throat is the main combination in study area, mediumporosity-medium throat and small porosity-thin throat combination are second, andthere are a less of small porosity-micro throat and micro porosity-micro throatcombination. Pore-cavity limestone reservoirs are the main type of reservoirs,porosity limestone reservoirs and fracture-porosity limestone reservoirs are second.Thin reservoirs are the main type of reservoirs in the interbred of anhydrite andlimestone (XVac), which have bad continuity and often distribute as lens. Medium-thick reservoirs, which have good continuity, are the main type of reservoirs in theupper of layered limestone (XVp), but thin reservoirs, which have bad continuity,are the main type of reservoirs in the lower of layered limestone (XVp). Thickreservoirs, which have good continuity and the trend which transforming from goodto bad when tracking from upper to lower, are the main type of reservoirs in Massivelimestone (XVm). The controlling factors of reservoir development are highstandsystem tract, rock types, sedimentary facies and diagenesis. Finally, through overallevaluation of reservoir and regional geological characteristics, the author believesthat the higher sections of region S are the most favorable reservoir areas, thesouthwest of well N5in region N, the higher sections of well M22, M23of region Mand Y are the less favorable reservoir areas.Gas fields discovered in the study area are all lithologic-structural combinedgas reservoir with normal pressure system except for the Y gas pool, a condensategas reservoir with abnormal low pressure system. Within these gas fields, thegas-water relationship is quietly complex, with different hydrodynamic system indifferent structure and even different in the same structure, resulting in no unifiedgas-water interface and gas column height. However, in general, the gas-waterinterfaces are high in the west and north but lower in the east and south. Based onthe geological condition and data set in the S gas field, with seismic prediction resultof reservoir development as constraints, virtual wells were implanted in region withrare wells as an interpolation control, then on the basis of logging data, stochasticmodeling method was adopted here to establish a reasonable geological modelcharacterizing the spatial and temporal variation rule of their reservoir propertyparameters.