## 2 Oil and Gas Geological Characteristics of the Basin
### Reservoir  
The discovered oil and gas reservoirs in the basin mainly include weathered Paleozoic basement, Triassic, Jurassic and Cretaceous, among which the Middle Jurassic is the most important reservoir. The Paleozoic reservoir is the weathered rock layer of the Lower Middle Paleozoic crystalline basement and granite intrusion. Taking the Oymasha Oilfield as an example, the granite oil-producing layer extends up to 300 m below the basement surface, and the tested production capacity is more than 2000 barrels/day [10].  
Lower Triassic reservoirs comprise coastal marine and lagoonal carbonates and clastics of the Olenekian stage and the Normaul Formation. The best reservoir properties are found in the sandstones of Unit A of the Normaul Formation, where open porosity values range from 8 to 18% and permeabilities can reportedly reach 1000 mD. Olenekian limestones and dolomites have porosities averaging 10% and low permeabilities (max 12 mD).  
Middle Triassic reservoirs are the most important in the Triassic section, with a burial depth range mainly between 3500 and 5000 m. They are represented by carbonates of the Aktas and Tenge formations, which were deposited in lagoonal-marine conditions. The Aktas Formation (Unit V) consists of solitic and reef limestones, although, in the south, they are interbedded with dolomites and tuffs. Unit V has high hydrocarbon flow rates and the lower parts of the unit have the best reservoir properties in the area, with the vuggy-porous nature of the limestones giving average porosities of 12-13% and permeabilities between 49 and 58 mD. Reservoir quality deteriorates in the upper part of the unit and fracturing becomes more common, with open porosities of less than 16% and permeabilities less than 9 mD [11].  
Upper Triassic reservoirs consist of a unit of coarse grain sandstones and conglomerates situated at the base of a sequence of variegated sandstones and siltstones of alluvial and lacustrine origin. The pay zone is relatively thin, with net thicknesses of 3 to 7 m. Porosities range from 9 to 16%, averaging 12%, and permeabilities are generally less than 50 mD.  
Middle Jurassic reservoir is the most important reservoir in the basin, including sandstones and siltstones from the Alinian-Callovian fluvial-deltaic to coastal phases, mainly fine- to medium-grained composite sandstones, with poor roundness and sorting of particles, and containing mud and calcareous cements. This set of reservoirs is a series of accretive and sub-accrete quasi-sequences, corresponding to sedimentary environments including river channels and channel fills, swamps, diversion mouth bars, delta lobes, and coastal phases. The reservoirs show heterogeneity both laterally and vertically, and a total of 24 producing layers have been identified, with an average net thickness of 10 to 30 m per producing layer.  
The sandstone and silty sandstone of the Hauterivian-Albian stage of the Lower Cretaceous are the products of shallow marine shelf sedimentation and can be divided into 7 reservoir units with an average porosity of 22% and a permeability of 90-970 mD. In addition, shallow marine sandstone reservoirs were developed in the Centoremanti stage of the Upper Cretaceous, with an average porosity of 25% and an average permeability of 310 mD.