Based on the molecular and stable carbon isotope compositions of over 60 gas samples collected from more than 10 exploration wells, in combination with the analytical data of source rocks and reservoir bitumen, the geochemical characteristics and origin of gases from the Puguang gas trap and other gas traps in Xuanhan-Daxian region of the eastern Sichuan Basin have been investigated. Results indicate that the natural gases in the Feixianguan and Changxing reservoirs of the traps belong to dry gas with high amounts of H2S, and show a molecular composition feature of the gas cracked from oil in the traps. The gases are dominated by methane with dryness index mostly more than 0.99, and contain abundant non-hydrocarbon components with CO2 and H2S being 5.32% and 11.95%, respectively. The methane is isotopically heavy (－29‰~－33‰ ), suggesting a high level of thermal maturity, and the δ13C values for ethane range mainly from －28‰ to －33‰ indicative of sapropelic organic source. The studied gases are believed to have a common source with the gases from the same reservoir age strata in the adjacent gas fields of the eastern Sichuan basin, but different in origin from Carboniferous gases due to different molecular and carbon isotope compositions. The compositions of biomarkers and carbon isotopes indicate a genetic relationship between the reservoir bitumen and the source rock in upper Permian Longtan Formation. The carbon isotope correlation between ethane in the natural gases and source rock kerogens shows that the gases in the reservoirs were mainly derived from Permian strata.
Aimed at the problem of no an effective method to evaluate the generation amount of low-mature gas quantitatively, this paper deduces, based on the formation mechanism of low temperature thermal thermogenesis of low mature gas, that the chemical kinetics theory, which has been applied widely and successfully in evaluating generation amount of normal thermal thermogeneicsis natural gas, remains to be an effective meothod of evaluating the low-mature gas generation amount of low-mature gaseous. On this foundation, the chemical kinetics models of representative kerogen samples of kerogen are constructed and calibrated, and in accompany with the kineticals models of resine and asphatene, which may be the precursor of low mature gas partly, they constitutes the evaluation meothod of low mature gas. It is the few minor compoenents with of low activation energy that results in the generation of low mature gas. The initial application indicates that the total generation amount of low mature gas is about 41 992×108m3 in north Songliao basin. In case of 1% of migration and accumulation coefficient, the resource amount of low mature gas is about 420×108m3，much lower than that of the normal thermal thermogenesisic natural gas. This suggests that it is workable to evaluate the generation amount of low mature gas using the chemical kinetic models and it can be also to popularize to other basins rich in low mature gas.
Microelements V and Ni are the main micro metallic element components in the petroleum. Because the material, energy and power foundations of upward migration exist, microelements V and Ni in oil gas pool source migrate vertically above and are distributed in the above environment. Using different methods to extract and analyze the V and the Ni content in soil, rock debris and formation water, we can know the V and Ni distribution characteristics and therefore may understand such information as oiliness in strata or the lower beds, the environment of deposition, the marine or continental deposit, the origin, and the oil source rock quality.
Based on the results of earlier studies on light hydrocarbons by several researchers, typical marine oils from the Tazhong area in the Tarim basin and typical coal-formed oils from the Turpan basin were selected and analyzed to find the differences of compositional characteristics of light hydrocarbons between the two oil categories and to provide new geochemical cognition for classifying the origin of oils. Through the GC-MS analytic technology, the light hydrocarbons data for two kinds of oils were obtained. (1) Light hydrocarbons of marine oils in the Tazhong area are rich in nC7, and the coal- formed oils from the Turpan basin are rich in MCyC6.(2) The Mango parameter K1 ranges from 0.97 to 1.19 in the marine oils, and it is consistent with the conclusion that Mango reported on light hydrocarbons based on a majority of crude oils data. But the K1 values of the coal-formed oils are particularly high (1.35～1.66) and far greater than marine oils. (3) Heptane values in the marine oils, from 32.3% to 45.4%, and isoheptane values, from 1.9 to 3.7, are respectively higher than the coal\|formed oils, which indicates that it is in a highly mature stage. (4) The expulsion temperatures for the coal-formed oils (96.8～102.2℃) are relatively lower than the marine oils (115.3～129.4℃).
The upper Paleozoic underwent complicated tectonic movement after the Indo-Chinese epoch in the Jiyang depression. The characteristics of the hydrocarbon generating evolution of the upper Paleozoic source rock are inconsecutive and multiphasic. Combining the tectonic-sediment evolution history and heated history, this paper researches the upper Paleozoic source rock in the Jiyang depression by contemporary petroleum geological theory. The upper Paleozoic source rock has obvious secondary hydrocarbon-generation characteristics. The first hydrocarbon generating period is late Middle Triassic，and the most important hydrocarbon generating period is late Early Teriary in which the source rock are highly and over mature. At the same time, four types of secondary hydrocarbon-generation models are concluded.
The log interpretation theory and technique about conventional oil and gas reservoirs are gradually becoming more and more feasible at home and abroad, but the research on condensate gas reservoirs is still weak. Firstly, with the analysis of electric characteristics of a condensate gas reservoir, we find that there are some differences between Tri-porosity logging amplitude, Deep Investigation Laterolog, Shallow Investigation Laterolog, and Microlaterolog, etc; secondly, in view of the feature of the condensate gas reservoir, the log interpretation model is built on the basis of the correlation of four reservoir parameters, and the reservoir grade scale is completed using conventional logging techniques combined with cross plot technique and the result of well testing; finally, according to the feature of Qiketai Formation and Sanjianfang Formation in the Hongtai oil field, we set up the reservoir classification criteria and divide the reservoir parameters into four types.
The north marginal basins of South China Sea are located at the junction of the Eurasian, the Pacific Ocean and the Indian-Australian plates. The geological setting is complex, the Tertiary sediment developed well, and petroleum geological phenomena are rich and colorful. Based on the previous research and exploration, the discovered oil and gas can be divided into three types, biogenic and sub-biogenic gas, normal mature oil and gas, highly and over-mature oil and gas. And there are also three types of non-hydrocarbon gas. The normal mature continental paraffin oil and gas are mainly distributed in the Beibuwan Basin and the Pearl River Mouth Basin. The highly and over-mature coal-related condensate gas oil and gas are mainly found in the Qiongdongnan Basin and some areas of the Pearl River Mouth Basin. Coal-related gas and non-hydrocarbo gas are mainly formed in mud diapiric zones of the Yinggehai Basin, the Eastern Qiongdongnan Basin and some areas of the Pearl River Mouth Basin.
Basins in Northern South China Sea belong to passive margin basins, and they are located at the junction of the Eurasian, the Pacific and the Indian-Australian plates, and also at the Superposed areas of Palaeo-Tethyan structural regimes and Palaeo-Pacific structural regimes. So the geological settings of these basins are complex, their Tertiary sediments developed well, and the phenomena of petroleum geology are rich and colorful. Based on the characteristics of framework and tectonics, the basins in Northern South China Sea can be classified into two types, strike-slip basins and rift basins. The source rocks of the former are mainly the well-developed, huge thickness of Neogene marine depression sedimentary rocks and Miocene marine sedimentary rocks; those of the latter are significantly the rift sediments of Paleogene continental facies, and large-scale source rocks of medium-deep lacustrine facies and inshore paludal facies with coal measure strata well-developed.
Yinggehai Basin is a youthful Cenozoic sedimentary basin with extension and transform. There are relationships between geotemperature-geopressure system, source rock thermal evolution and migration & accumulation of natural gas. Based on the analysis of the distribution of the geopressure field and the geotemperature field, there are two types of geotemperature-geopressure systems in the basin: composite geoperature-geossure system of high pressure in the central sag and singleness of temperature-geopressure system in the Yingdong slope. The controlling function of different geotemperature-geopressure systems on migration and accumulation of natural gas were analyzed. The vertical migration and accumulation of natural gas are under the control of conducting system and composite temperature-pressure system of high pressure, and the singleness of geotemperature-geopressure system and feebleness of vertical hydrodynamic force are the main reasons for lateral migration of the gas. There are important relationships between the migration of natural gas and the geotemperature-geopressure systems in the Yinggehai basin.
The Jiyang sag is a Meso-Cenozoic faulting basin developed in the North China platform, whose stress field is of poly-phase, multi-direction and complexity because of strike-slip together with large-scale rifting basinogenesis in eastern China, as well as collision and subduction between the India-Eurasia continent and the paleo-Pacific plate since late Mesozoic Era. Faults in this basin are developed very well with a polyphased and long-termed active nature. Geometrically, there are four kinds of fault lateral combinations: parallel, echelon, brush and oblique faults. And the faults can be classified into reverse, negative and positive inversion and flower faults. According to the scale and its control over tectonic, sedimentary and hydrocarbon migration and accumulation, the faults can be orderly divided into five kinds: basin-controlling, subsag-controlling, depression-controlling, fans-controlling and sandbody-controlling faults. As a superimposed sagging basin, its major oil-gas reservoirs are anticlines or fault-nose structures, which are mainly composed of “y” and “anti-y” shapes of compound fault series. Faults of various scales play different roles in controlling depression, sedimentation and hydrocarbon accumulation. The syn-sedimentary fault plays a very important role in both formation of sag and migration and accumulation of oil and natural gas.
The formula between strike-slip rate and subsidence rate in the pull-apart basin is derived by doing theoretical model studies. The result shows that the strike-slip moving rate has a steady relationship with the geometric parameters, the maximal subsidence depth and the subsidence rate. The Weibei sag in the Laizhou Bay is selected as the study area. Four wells in different structural positions of the Weibei sag are chosen to do subsidence history reconstruction using the back-stripping method and the empirical relationship between Tan-Lu fault strike-slip rate and Weibei sag subsidence rate is obtained. According to the calculating results of the wells Wei 1 and Yang 5 in the north depression, it is concluded that the Cenozoic dextral strike-slip displacements of the Middle Tan-Lu fault in the Laizhou Bay is about 40km.
Research of the T3x2 tight gas reservoir of Neijiang-Dazu Region in the transitional zone of Chuanzhong and Chuannan reveals that high capacity wells are in zonal distribution and are closely related to palaeostructures. In the process of deposition, because of the water bodies with high-energy on both sides of the palaeotectonic belt, the higher maturity coarse particles were deposited. In the diagenesis, cracks were formed by differential compaction. These make the top and two wings of the palaeotectonic belt to be favorable oil and gas accumulation zones. Due to the relatively poor conditions of sedimentation and diagenesis away from the palaeotectonic belt, physical properties of the reservoirs became poor, so that a lithologic reservoir was formed on the palaeotectonic belt.
Reservoirs of the Cretaceous Bashijiqike Formation in the Dabei area of the Kuqa depression are dominantly lithic sandstone with a little feldspar-lithic sandstone, and mostly fine-middle grains, with 2%~10% iron-mud and mud, 3%~15% calcite cement. Clay minerals are mostly illite, chlorite, and illite-smectite (15%~20% smectite), and have no kaolinite. The main pore types are emposieu-primary pores (50%~90%), followed by micropore and tectoclase, and the reservoirs generally belong to low porosity-permeability and extra-low porosity and permeability. Comprehensively analyses show that episodic thrusting-overriding tectonism had an utmost importance in controlling the sediment cycle, alteration of water salt and distance from provenance. Origin mechanisms of extra-low porosity and permeability reservoirs are: poorly sorting middle-and coarse-stones formed in fan delta front during tectonically active time, high content debris formed by proximal and quick deposit, high carbonate cement formed in dry saline lake, relatively deep burial in early diagenesis and quick deep burial and compaction in later diagenesis. High productive reservoirs are the synergistic effect of well sorting fine-and middle-stones formed in braided river delta front during tectonically calm time, early regressive corrosion and poor carbonate cement, later tectonic disruption.
In Mesozoic,the Ordos basin is a down warped lake basin sediment in a typical stable Craton. In the Triassic Yanchang period,the margin of the lake basin ramp belt (northern of the line Wuqi-Ziwuling) developed sedimentary slope breaks, however,using seismic reflection characteristics and lithological stratum correlation techniques are difficult to identify it. Within a three-sequence-stratigraphic framework, this paper identifies the margin of the ramp belt's sedimentary slope breaks of the Yanchang period's basin from three aspects: sequence stratigraphy filling shape, sedimentary microfacies association types,short-term base-level cycle superposition types,and the corresponding ratio (A / S) of the accommodation space (A) and the supply of sediment flux (S). On this basis, an identification method of the margin of the ramp belt's sedimentary slope breaks of down warped lake basins and its recognition model are established.
The formatin of interest is the Lower Crataceous. Because of the complicate terrains and strata, the singal to noise ratio is very low. The interpreted layers have tremendous differences from what are drilled. This paper uses spectromery date to distingish the successions and predict the hydrocarbon potential. The spectrometry log does not only provide the total gamma ray of the formation, but also the contents of Uranium, Thorium and Kalium. The characteristics of the natural ray spectrometry log can reflect the radioactivity of sedimentary, and by the analysis of the correlated characteristics among them, we can recover the ancient depositional environment. By handling the digital information of the natural gamma ray spectrometry log, the curve of Th/U, U/GR can be gotten. The Th/U curve linked with the regional data indicates the weathered surface, and the U/GR curve indicates the changes of the environmental water. So the systems tracts in a certain area can be identified. On the basis of the systems tracts, the stratigraphical correlation and stratum prediction has got a good effect in the application of the Wushi area.
According to core and thin-section observation,drilling,logging and reservoir physical property analysis, the reservoir body in the Xujiahe Formation of the southwest Sichuan basin is braided delta front sandbodyies which have wide distribution, poor physical properties, and low porosity and low permeability. The lithology is mainly feldspar quartz sandstone and lithic quartz sandstone, and the main pore spaces are relict primary intergranular pores, intergranular solution pores, intragranular solution pores, pores in grain films and fractures. Reservoirs of the Xu 2 and Xu 4 are well developed and the Xu 2 reservoir physical properties are better than Xu 4. Favorable reservoirs were underwater distributary channels and mouth bars. High quality reservoirs are controlled by structural conditions, sedimentary facies and particle size of rocks. The types of reservoirs in the Xujiahe Formation are mainly Types Ⅱ and Ⅲ, and the type I is less.
By analysis and study of bulk of wells and outcrops, on the basis of generalizing various structures and stratigraphic classifications of Cambrian strata of different periods in south Sichuan basin, in terms of lithostratigraphic characteristics and comprehensive analysis of its paleontologic characteristics and rock resistivity, the Cambrian strata in south Sichuan basin can be divided into four formations: Jiulaodong and Longwangmiao Formations of Lower Cambrian, Gaotai Formation of Middle Cambrian, and Xixiangchi Group of Middle-Upper Cambrian. Study on the stratigraphic characteristics and distribution indicates that the Jiulaodong Formation of Lower Cambrian mainly develops clastic rock with little carbonate rock, the Longwangmiao Formation of Lower Cambrian mainly develops marine carbonate, the Gaotai Formation of Middle Cambrian develops mixing deposit of clastic and marine carbonates, and the Xixiangchi Group of Middle-Upper Cambrian develops marine carbonates. The thickness of each Cambrian Formation (Group) in the study area decreases gradually from southeast to northwest.
The nose structure is an important location for hydrocarbon accumulation. Based on the analysis of its geological background and oil geological factors, the basic features of Cenozoic hydrocarbon accumulation in the Honggouzi structure is discussed. Firstly, the structure undergoes depression, extrusion and finalization in the front of the Altun Mountain since the Cenozoic, and abundant source reservoir seal assemblages which has good space-time association are formed. Secondly, the structure develops two types of oil-gas reservoirs: crack-pore type which is distributed in the east of the structure, and pore type which is distributed in the superficial layer of west structure.
By the observation and description of field geological profiles and cores, sand is divided into four members in the Lower Ganchaigou Formation (E13) in the Hongliuquan area of the Qaidam basin. This paper analyzes the sand members I and Ⅱ. Through integrating sedimentary microfacies of the profile as well as content and distribution of the microfacies sandstone, ichnographies of the sedimentary microfacies in the area are protracted. Based on the analysis of electrical characteristics of the sedimentary facies, the pattern recognition of the sedimentary microfacies and the analysis of single-well facies, the sedimentary succession and the model of sedimentary evolution are established. The results show that subaqueous channel sandbodies and sheet sandbodies of deltafront are well developed in the sand members I and Ⅱof the Lower Ganchaigou Formation. Through the study of sedimentary microfacies and evolution, favorable reservoir facies are found in the area, consequently providing a geological foundation for progressive exploration in the oilfield.
In the process of fine reservoir description of the Gao-17 fault block, GaoQing oilfiled, we discovered crevasse channel microfacies, and then compared and analyzed its origin, sedimentary characteristics, depositional model, and spatial distribution. The crevasse channel is an extreme narrow and shallow short range small-size channel with fixed paths and specified metering function in wide interchannels of the fluvial-delta sedimentary system, and is formed after the main stream channel rip-up in the cataclysm period. The formation of the crevasse channel impacts the original main stream channel little. In contrast with the main stream channel, the crevasse channel presents features of fine-grained, poorly graded, high silt content, small bedding scale, weak erosion, a little normal cycle, a little bad reservoir nature, very thin sand body (<2.5m) and narrow (<100ｍ),etc. By means of analyzing sub-sea crevasse channel depositional mechanism, we established 4 depositional models including fading model, branching model, extreme fan model and affluxion model. Because the sub-sea crevasse channel is very narrow and discrete, it develops small and plenty remaining oil concentration sand body of uncontrollable well network or imperfect injector producer. And because the sand body scalar quantity is very large, reserves of remaining oil are extraordinary impressive.
Ordovician carbonate is an important exploration target in the Hetian River Gasfield and its adjacent areas. Natural gas has been found in the region. Controlled by karst, cracks and sedimentary environments, the reservoir pore structure in Ordovician carbonate is very complex, and dissolution caves and cracks are the most important accommodation spaces. The petrophysical properties of various Ordovician stratigraphic units are different. The dolomite in the buried hill of Penglaiba Formation is the best reservoir, and the limestone of Lianglitage Formation is the worst reservoir, and Yingshan Formation is in the between. Three stages of weathering crust karst and five stages of structural fractures developed in the Hetian River Gasfield. Multiple stages of structural fractures (especially the structural fractures developed during the Himalayan movement), posttectonic karst and paleo-weathering crust karst have influenced and spliced each other. These factors and the sedimentary environment controlled the complicated karst system of the Hetian River Gasfield, and they effectively improved reservoir pore space of Ordovician carbonate together.
The discovered Cambrian dolomites in Milan well 1 and Yingdong well 2 are important reservoir rocks in the carbonates of the Lower Paleozoic, Eastern Tarim basin. The high-quality dolomite reservoir rocks mainly occurred in carbonate shallow ramp deposits. Geochemical analyses of carbon and oxygen isotopes, ordering determination and inclusions suggest that the dolomites may be divided into two types: the first is the high temperature hydrothermal fluid dolomitization, the second is the seepage reflux dolomitization. The high temperature hydrothermal fluid dolomitization is crucial to the formation of high-quality dolomite reservoir rocks. The formation of high-quality dolomite reservoir rocks is the result of the seepage reflux dolomitization being overprinted by the high temperature hydrothermal fluid dolomitization in the region.
Donghe Oilfield of the Tarim Basin is an ultra deep sandstone reservoir. The compaction of formation rock is good. According to prediction study and pilot tests, it would not produce sand. However, after it was put into production, sand production gradually occured, and became more and more serious year by year, owing to the significant reservoir pressure drawdown and water-cut increase. How to dynamically predict the sand production according to the changing reservoir conditions during the reservoir development is a key issue both in theory and in practice.This paper proposes two modification factors and which reflect the influence of pressure drawdown and water-cut increase respectively. Using the revised formula, the calculation results and prediction conclusions match the reservoir situations very well. The proposed new idea and method provide a valuable reference to sand control of other sandstone reservoirs.
Horizontal well development is an important technology for increasing production and improving development benefit. The horizontal well geosteering technology includes: the optimization of well sites which guarantees that construct of horizontal wells, reservoir beds and off-take potential are all substantive, so as to reduce the horizontal well performance risk; the optimization of horizontal well track design, to predict accurately target points and the following target points based on the K1 construct, the stratum thickness and effective thickness of Majiagou team mbrs, the construct of K2 and the top of the Ordovician, and stratum thickness and dip angle, so as to ensure the security of track design; the optimization of geological orientation tracking analysis, to insure targeting according to the coal bed and the top of the Ordovician drill-met, and to forecast the following target points by combining debris, drilling times, gas survey, GR along with drilling, stratum thickness and dip angle. The application in the Lp1 well achieved high production after acidification.
Qinghai Sebei gas field is a shallow biogenetic gas field in the Quaternary formation, and its reservoirs are easilyprone to be deformed because of unconsolidated sandstones and poor lithologiesy, which results in obvious reduction of porosity and permeability during the process of pressure change. By overburden pressure experiments, porosity and permeability variation characteristics of Sebei unsolidated sandstone reservoirs isare analyzed. The relationship of porosity and effective stress is binomial. Dimensionless permeability reduces very quickly at the low pressure stage; however, it reduces slowly atat the high pressure stage. And at these two different stages, the relationships of permeability and effective stress are different, the former is exponential and the latter is the relationship of permeability and effective stress of is power. At the later period of gas field development, porosity will reduce by 10 percent, for gas reservoir pressure reduction and permeability will reduce to 0.1~0.6 times of original values and , whichit wwill influence gas production greatly-reduce gas well productivity by 47.1 percent. Simulation results show that, when rock deformation is considered during gas field development, the stable production period will reduce 3 years and recovery percent of reserves at the end of the stable production period and at 30 years will reduce respectively by 6.45 percent and 5.47 percent, respectively.
Pore pressure decreases, effective stress increases, and permeability declines due to matrix and pore compression as gas produces in low-permeability gas reservoirs. This affects the deliverability of low-permeability gas reservoirs. Based on Darcy flow law and the universal cognition that the rock permeability varies with the effective stress exponentially, this paper presents a new equation considering rock permeability stress-sensitivity for estimating the productivity in low-permeability gas reservoirs. A case calculation with this new equation shows that considering permeability stress-sensitivity in the prediction is necessary, especially for the high stress-sensitive low-permeability gas reservoirs,and this will provide necessary reference to determining reasonable producing pressure drop and producing rate.
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