Search

Based on core data, physical properties, well logging, gas testing, composition, and helium distribution from 13 newly drilled wells, in combination with regional tectonic evolution, depositional background, and typical gas reservoir profiles, helium-rich gas reservoirs geology characteristics and main controlling factors have been explored. The results show that three gas reservoirs formed from bottom to top, i.e., granite and metamorphite fractured reservoir in the basement, tight sandstone structural reservoirs in the Devonian, and huge continuous carbonate reservoirs in the Lower Carboniferous. Fractures developed on structural points control high-production, and gas is prominently displayed in low structural areas. The dry gas exhibits richness in helium, with decreasing helium content from the basement to the Carboniferous. The pre-Devonian large-scale granite and metamorphite basement serve as the primary helium source, while the high-gamma sandstones of the Devonian and the Carboniferous act as secondary sources. Fractures play a dual role, being filled up and conducting in two ways: gas from the Carboniferous source rocks migrates downward, and helium migrate upward. Thick interbedded gypsum-tight limestone on top of the Lower Carboniferous act as the regional caprock, effectively preserving the helium-rich gas reservoirs. [ABSTRACT FROM AUTHOR]

The Dabei Gas Field is a recently discovered giant tight-gas field in the Kuqa Subbasin, western China. The reservoir porosity and permeability mainly range from 1% to 8% and from 0.01 to 1 md, respectively. The hydrocarbon (both gas and light oil] accumulation processes in the tight-sandstone reservoirs were studied based on detailed reservoir characterization, thermal maturity of both gas and light oil, hydrocarbon charge history, regional tectonic compression, and thrusting. Two episodes of oil and one episode of natural-gas charge were delineated in the tight-sandstone reservoir, as evidenced by [1] similar sources but different maturities for the gas and light oil, (2] the presence of abundant bitumen in the tight-sandstone reservoir, (3) the presence of both hydrocarbon gas inclusions and oil inclusions with two distinct fluorescence colors, and (4) the presence of two groups of aqueous inclusions (coeval with the petroleum inclusions) with contrasting homogenization temperatures and salinities. The oil inclusions with the blue-white fluorescence color were determined to have been trapped at 5-4 Ma, whereas the gas charge may have occurred at circa 3-2 Ma, corresponding to a salinity change recorded in the aqueous inclusions. The hydrocarbon accumulation processes appeared to be controlled by the tectonic compression of the South Tianshan Mountains. Intense tectonic compression caused thrust fault reactivation, which provided pathways for hydrocarbon migration. Overpressure evolution of the reservoir indicates that an intense tectonic compression began at circa 5 Ma, which caused thrust activation and concomitant oil charge into the relatively porous part of the reservoir. Subsequent tectonic compression caused uplift and erosion associated with thrusting at the end of the Kuqa Formation deposition (ca. 3 Ma), with thrust faults and fractures acting as major migration pathways for the gas accumulation in the already-tight sandstone reservoir resulting from both compaction and tectonic compression. [ABSTRACT FROM AUTHOR]

Kela-2 is a giant gas field with a proven reserve of 597 tcf in the Kuqa depression, northern Tarim Basin. Widespread overpressures have been encountered in the Eocene and Cretaceous sandstone reservoirs of the field, with pressure coefficients up to 2.1 from drill-stem tests and well-log data analysis. Disequilibrium compaction associated with horizontal tectonic compression may be the dominant overpressure mechanism in the sandstone reservoirs, because the overpressured sandstone with a maximum burial depth over 6000 m (19,685 ft) displays anomalously high porosity and low density. The causes for sandstone reservoirs with anomalously high porosity in the Kela-2 gas field were studied based on an integrated investigation of sandstone reservoir characteristics, paleo oil-water contact, petroleum charge history, and overpressure evolution. Collective evidence indicates that early oil charge had retarded the porosity reduction of the reservoir sandstone and resulted in disequilibrium compaction from overburden rocks, and overpressure from disequilibrium compaction and horizontal tectonic compression at the beginning of the rapid subsidence and deposition in the Kela-2 gas field again contributed to the preservation of the reservoir porosity: (1) overpressured mudstones in the Kela-2 gas field are characteristic of normal compaction, and overpressure was generated by horizontal tectonic compression instead of disequilibrium compaction; (2) the reservoir sandstones with high porosity and permeability are associated with high paleo oil saturation, as indicated by quantitative grain fluorescence (QGF) responses and anomalous QGF on extract intensity; (3) sandstone units below the paleo oil-water contact have very low porosity and permeability; and (4) three episodes of oil and one episode of gas charge are identified in the sandstone reservoirs of the Kela-2 gas field, and the later two episodes of oil charge occurred circa 5.5--4.5 Ma, which corresponds to the beginning of the rapid tectonic subsidence and deposition in the Kuqa depression. The initially charged oil in the sandstone reservoirs was subsequently displaced by gas at circa 3-2 Ma through fault activation at the edge of the anticline trap. The overpressure evolution for the K1bs reservoir sandstone in the Kela-2 gas field indicates that the apparent overpressure development in the sandstone reservoir began at 5 Ma following the major oil charge and has been maintained to the present. Overpressure development from 5 Ma in the sandstone reservoirs of the Kela-2 gas field is believed to be the dominant cause of the porosity preservation. [ABSTRACT FROM AUTHOR]

Abstract: The Kela-2 gas field, located in the Kuqa Depression of Tarim Basin, is the most productive gas field in China, based on per unit surface acreage. The gas trap is a subsurface anticline formed along an E–W trending thrust belt in front of the southern Tianshan Mountains. The gas pay zones are located in fine-grained sandstones of a fan delta front facies in the Lower Cretaceous Bashijiqik Formation (Klbs), and Eogene evaporitic rocks serve as regional cap rocks. The gases were derived primarily from over-mature Jurassic coal measures. Peak gas generation and final shaping of the Kela-2 structure occurred relatively late, in the late Himalayan orogeny (5–0Ma). Thrust faulting appears to have played a significant role in the hydrocarbon generation, migration, accumulation and preservation in this foreland basin. Detailed delineation of the Kela-2 gas field indicates that high-quality regional cap rocks are essential for the formation of the super-rich, abnormally pressured, giant gas fields, while buried anticlines under the overthrust zone are the favorable gas habitat. Late gas accumulation is considered to be decisive for the conservation of the giant gas fields. [Copyright &y& Elsevier]

Hetianhe is a big carbonate gas field which is found and demonstrated in the period of "Chinese National Ninth 5-Year Plan". The proved reserve of Hetianhe gas field is over 600x10&sup8; m³. Its main producing layers are Carboniferous bioclastic limestone and Ordovician carbonate composed of buried hill. The former is stratified gas pool with water around its side, and the latter is massive gas pool with water in its bottom. The gases in the gas pools belong to dry gases with normal temperature and pressure systems. Based on the correlation of gas and source rock, the gases are mainly generated from Cambrian source rocks. According to the researches on source rock and structure evolution, and the observations on the thin section to reservoir bitumen and the studies on homogenization temperature of fluid inclusions, the gas pool has been identified and divided into three formation periods. The first is Late Caledonian when the oil generated from the Cambrian source rocks and migrated along faults, as a form of liquid facies into Ordovician carbonate reservoir and accumulated there. After that, the crust uplifted, the oil reservoir had been destroyed. The second is Late Hercynian when condensate gases generated from the Cambrian source rocks and migrated into Ordovician reservoir, as a form of liquid facies. Since the fractures had reached P strata, so the trap might have a real poor preservation condition, and the large-scale gas pool formation had not happened. The third gas reservoir formation period occurred in Himalaya. The fractures on both sides of Hetianhe gas field developed violently under the forces of compression, and thus the present fault horst formed. The dry gases generated from Cambrian source rocks and migrated upwards as the form of gas facies into Ordovician and Carboniferous reservoirs, and the large gas pool as discovered at present was formed finally.

Reports that the Kela-2 gas field is located in the center of Kelasu structural belt in Kuqa Depression. Fact that this trap is one of a series of traps in the folded belts which are distributed in a string of pearls in the dual structure; Statement that the primary gasbearing layers are sandstone of Lower Cretaceous K&sub1;bs while the secondary layers are dolomite member and glutenite member of Lower Tertiary E&sub1-2;km and sandstone of Lower Cretaceous K&sub1;b; Reference to the fact that the main component of natural gas is methane whose content is higher than 97%; Characterization by dry gas whose source rock is Jurassic coal measure; Formation of the Kela-2 structural trap during the Xiyu period, becoming a reservoir in the late time; Formation of late and the thick seal rock of Lower Tertiary gipsmantle in the reservoir as the avail reason why the giant Kela-2 gas field has been well kept; Conclusion that the abnormal high pressure of the Kela-2 gas field results from the strong structural compression in the northern part during the Xiyu period.

Reports that the Kuqa depression is a foreland basin developed with Mesozoic-Triassic-Jurassic coal-bearing formation. Indication of the research results of the coal-derived hydrocarbon foreland basins in Kuqa depression that the coal-bearing formation can be the rich sources for generating gas because of their thickness and rich source rocks with gas-generating predominant kerogen; Statement that although the foreland thrust belt mainly acting in compression is very complicated, integral large structural traps can be formed; Claim that the thrust belt can act as the passage for communication with deep source rocks; Development of the high quality gypsolish and gypseous mudstone cap rock in the upper formation as the key for the formation of the large gas field; Dependence of the late formation of reservoirs in the large gas fields on the hydrocarbon-generating history controlled by the foreland basin and the developing process of foreland thrust belt.

Reports that there is a gas-rich and well-charged petroleum system in the Kuqa Depression where Triassic and Jurassic source rocks play important roles. Distribution in an area of more than 10000 km² and with a thickness of up to 1000 m; Composition of dark mudstones, carbonaceous mudstones and coal seams containing 6%, 40% and 90% of TOC, respectively, mainly of humic organic matter; Statement that, as high-quality regional cap rocks, the Neogene and Eogene gypsum rocks and gypseous mudstones matched well with the underlying Neogene and Cretaceous-Eogene sandstones; Formation of the most favorable reservoir-seal assemblages in the Kuqa Depression; Formation of the Jurassic sandstones and mudstones into another favorable reservoir-seal assemblage; Shaping of the traps late in the fold-thrust belt, mainly fixed in the Tertiary-Quaternary, where ten structure styles have been distinguished; Reference to tge fact that these traps spread as a zone in N-S, are scattered like a segmental line in W-E and show tier-styled vertically; Statement that the best traps are gypsum-salt covered fault-bend anticlines related to the passive roof duplex; Characterization of this petroleum system by late accumulation; Distribution in the early Himalayan Movement of an accumulation of mainly gas condensate and oil in the outer circular region of the kitchen; Claim that in the middle and late Himalayan the gas accumulations mainly formed and were distributed in the inner circular region near the kitchen; Statement that the overpressure of gas pools is common and is formed by seal capacity of thick gypsum layers, extensive tectonic compression and large uplift; Suggestion that the well-preserved anticline traps underlying the high-quality regional cap rocks of the Tertiary gypsum rocks and gypseous mudstones are the main targets for the discovery of giant and medium-sized gas fields; Importance of the above conclusions for the petroleum geology theory and the exploratio