Abstract Hydrocarbon exploration in China has recently expanded to deep tight oil and gas from the Lower Jurassic sandstone reservoirs in the central Junggar Basin. These deeply buried sandstones (>4.5 km) have generally been overpressured due to hydrocarbon saturation, but have not been intensively investigated in terms of diagenesis and reservoir quality. The Lower Jurassic sandstones are feldspathic litharenites (average Q 28 F 29 R 43 ) and were deposited in the lacustrine–deltaic environments of the high-palaeolatitude continental basin under warm, humid climatic conditions. Progressive burial with only slight uplift of the strata led to a relatively stable burial–thermal regime, resulting in minor diagenetic changes consisting of compaction, cementation by carbonates, quartz and clay minerals, and partial dissolution of feldspars and rock fragments. Despite generally similar diagenetic histories, the studied sandstones reveal considerable variations in the development of pore systems, porosity and permeability. The best reservoirs are represented by medium- to coarse-grained depositional facies with higher porosity (average 8.43%) and permeability (average 0.59 mD). All the evidence suggests that integrated consideration of the roles of depositional settings and diagenesis, linked with burial–thermal history and overpressure evolution conditions, provides a helpful means of predicting reservoir quality.

Abstract Carbonate cements in the early Miocene Temblor Formation at Kettleman North Dome oil field, on the western flank of the San Joaquin basin in California, formed in marine and mixed meteoric-marine pore waters. Arkosic sands were deposited in deltaic to shallow marine and deep marine environments. Carbonate cements preserve the degree of compaction at the time of cementation. Micritic calcite cements are interpreted to have formed at the sediment-sea water interface when intergranular pore space was about 40%. Later, dolomite cements formed during shallow burial, when intergranular porosities were about 30%. Coarse crystalline calcite cement and grain-replacement cement precipitated during deep burial, when intergranular porosity was less than 25%. These carbonate cements originated from three types of formation waters based on oxygen isotopic data. The micrite formed in nearly pure meteoric water at the sediment-water interface. The dolomite precipitated from mixed marine-meteoric water during shallow burial. Late calcite that formed during deep burial (70–120°C) precipitated from diagenetically modified marine waters. Studies of six fields in the central and eastern San Joaquin basin indicate that carbonate cements originate from both marine pore waters and from meteoric incursion during deposition and uplift of the basin perimeter. Sediments deposited in non-marine to shallow marine environments in the basin flanks were subjected to meteoric water infiltration during shallow burial. Early carbonate cements with meteoric isotopic signatures are found at distances of up to 5 km (Round Mountain Field, eastern flank) to 15 km (North Kettleman Dome Field, western flank) from potential recharge areas. Meteoric recharge is also recorded late in the cement history, due to locai uplift on the west flank. The uplift focused meteoric water into deep marine sands up to 15 km from the basin edge (North Belridge Field). In the central basin, sands deposited in deep marine environments were isolated from meteoric influence due to their distance from meteoric recharge areas and lack of hydraulic continuity with the basin flanks. Thus, these sands only contain carbonate cements with marine or evolved marine geochemical signatures (e.g., North Coles Levee Field).