| Bachu-Maigaiti area is in the west-central part of the Tarim Basin and enclosestwo second-ordered structural units: Bachu Ridge and Maigaiti Ramp. BoreholesQun6, BT4, and BK8, which are drilled in the Devonian Reservoirs by PetroChinaand Sinopec, have shown indication of hydrocarbon. Exploration in theSilurian-Devonian strata of the Bachu-Maigaiti area has been low, resulting ininsufficient basic datasets and unclear relation between the structural evolution andreservoir formation. Therefore, it is significant to conduct detailed research on thereservoir formation mechanism of the Silurian-Devonian strata of theBachu-Maigaiti area, which would shed light on some dilemmas during the prospectin this area and serve as key analog for other similar petroliferous basins.Based on detailed studies on outcrop, well logs, seismic data, petrography, andpetrophysical properties, this research focused on facies analysis in the first place.Reservoir formation mechanism is then tentatively analyzed, followed by studies onreservoir type, hydrocarbon distribution, and controlling factors on hydrocarbonaccumulation. Upon the completion of basin modeling and flow inclusion analysis,reservoir formation stages are divided, reservoir model is constructed, and reservoirformation pattern is summarized.The Silurian-Devonian strata in the study area was deposited in coastal shallowmarine environments, and consists of three major facies (i.e., coastal, tidal flat, anddeltaic), ten subfacies, and several microfacies types. Based on reservoir formationanalysis, the Silurian-Devonian strata in the study area do not contain thickdark-colored mudstone, and therefore, are not source rocks. Hydrocarbon is sourcedfrom two sets of marine strata: Cambrian-Ordovician and Carboniferous-Permianstrata. The Devonian reservoir rocks are mainly composed of quartz sandstone, lithicquartz sandstone, and lithic sandstone, and are characterized by low compositionaland structural maturity. Pore types are mainly secondary dissolution pores. Regionalseals in the study area include two sets of strata. One is the Upper SilurianYimugantawu Formation mudstone, which is characterized by reddish and brownishcolored, stable thickness, wide and continuous distribution, and well-developeddiagenesis. The other one is thick Bachu Formation mudstone interbedded withgypsum mudstone and silty mudstone.Based on studies on hydrocarbon distribution and controlling factors of thehydrocarbon accumulation, reservoirs in the Bachu-Maigaiti area locate on thejunction of the updip part of the ramp and local structural belt of the rise and arecontrolled by these two factors as well. By analyzing failed wells, controlling factorsof the hydrocarbon accumulation are summarized, including ancient rise and ramp, fractures, unconformities, matching relation between trap and source, and facies.Based on burial and thermal history and fluid inclusion analyses, reservoirformation of the study area is divided into three stages. The first stage (268-252Ma,latest Permian), during which the latest Hercynian tectonics occurred, formed thepatch ways for hydrocarbon to migrate in a large scale. The second stage (12.5-7Ma,middle Tertiary), which shows a uniform fluid inclusion temperature of107.8-114.7℃, is mainly characterized by the migration of highly maturedhydrocarbon. The third stage (6-2Ma, latest Tertiary to late Quaternary), whichshows a fluid temperature of130.8-137.7℃, is mainly characterized by highlymatured natural gas.Finally, four reservoir formation models are proposed. The first faultedanticline model involves faulted anticline traps composed of at least one fault.Reservoir in this type has irregular shapes and relatively small areas. Faults aremajor migration path ways for hydrocarbon. The second ancient structural remainingmodel is characterized by the path ways of initial faults. In this model, hydrocarbonmigrates through this path way upward into effective traps to form ancient reservoirs.After the Himalayan tectonic activities, the ancient reservoirs are partly altered,resulted in the modern reservoir. In the third magmatic thrusting model, magmaticintrusion during the late Hercynian altered part of the initial anticline traps. Theresultant contact metamorphic belt served as lateral barriers for hydrocarbonmigration. The fourth model deals with the injection of late natural gas, andcorrespond to the migration of the highly matured natural gas during the lateHimalayan. Reservoir in this model is mainly in areas where fractures are poorlydeveloped and petrophysical properties and area of sand bodies are decent. |
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