Lithologic Trap Research Articles

The pre-Jurassic Meng-Shaan paleochannel and its control on the accumulation model of Yan 10 reservoir in Central Ordos Basin

In the setting of the increasing density of exploration wells, and the decreasing scale and increasing difficulty of discovering Jurassic paleogeomorphology reservoirs, it is urgent to deepen the fine depiction of pre-Jurassic paleogeomorphology features and to analyze their controlling effect on reservoirs. Based on abundant logging data combined with indoor microscopic observation and experimental testing, the paper applies the theory of reservoir configuration analysis for the first time to the single-channel period division of the pre-Jurassic Meng-Shaan paleochannel which deposited with the braided channel sand bodies. Meanwhile, it reveals the controlling effect of hydrocarbon accumulation and the distribution characteristics of the Yan 101 reservoir as ' paleogeomorphology and sedimentary facies combination' to determine the type, 'migration channel' to determine the distributing range, and 'low-amplitude structure' to determine the trap in the study area, and also the reservoir accumulation modes in different channel ranges have established. In conclusion, the single-channel boundary of the Meng-Shaan paleochannel in different periods controls the path of oil and gas migration, thus controlling the distribution range and reservoir type of the Yan 10 reservoir. Moreover, depending on the production data we derived that it should prefer the composite trap reservoir of the Fuxian period and the Yan 102 period, as well as the structural reservoir on the periphery of the Yan 102 period single-channel for the further exploration of the Yan 10 paleogeomorphology reservoirs within the development range of the Meng-Shaan ancient river. In particular, the lithologic trap reservoirs within the Fuxian period channel and the Meng-Shaan main channel, such as the reservoirs of the 'Source' of the secondary channel type and the paleochannel type, which could as a replacement accumulation model for increasing reserves and production.

Regional unconformities and their controls on hydrocarbon accumulation in Sichuan Basin, SW China

Based on outcrop, seismic and drilling data, the main regional unconformities in the Sichuan Basin and their controls on hydrocarbon accumulation were systematically studied. Three findings are obtained. First, six regional stratigraphic unconformities are mainly developed in the Sichuan Basin, from the bottom up, which are between pre-Sinian and Sinian, between Sinian and Cambrian, between pre-Permian and Permian, between middle and upper Permian, between middle and upper Triassic, and between Triassic and Jurassic. Especially, 16 of 21 conventional (and tight) gas fields discovered are believed to have formed in relation to regional unconformities. Second, regional unconformity mainly controls hydrocarbon accumulation from five aspects: (1) The porosity and permeability of reservoirs under the unconformity are improved through weathering crust karstification to form large-scale karst reservoirs; (2) Good source-reservoir-caprock assemblage can form near the unconformity, which provides a basis for forming large gas field; (3) Regional unconformity may lead to stratigraphic pinch-out and rugged ancient landform, giving rise to a large area of stratigraphic and lithologic trap groups; (4) Regional unconformity provides a dominant channel for lateral migration of oil and gas; and (5) Regional unconformity is conducive to large-scale accumulation of oil and gas. Third, the areas related to regional unconformities are the exploration focus of large gas fields in the Sichuan Basin. The pre-Sinian is found with source rocks, reservoir rocks and other favorable conditions for the formation of large gas fields, and presents a large exploration potential. Thus, it is expected to be an important strategic replacement.

Development model of shallow lithologic traps and natural gas accumulation mechanisms in marine deep-water areas: A case study in the qiongdongnan basin, south China sea

Although hydrocarbon gases are widely distributed in the shallow strata of marine deep-water areas, it is generally believed that these shallow gases have little potential to form large gas accumulations due to unfavorable conditions, especially for effective trap development. Based on a comprehensive analysis of the latest geological exploration data of the deep-water areas of the Qiongdongnan Basin, this study reports a large lithologic trap that developed in the shallow Ledong Formation with burial depths of 200–600 m below the seafloor (mbsf). Systematically, this work investigates the development model of the trap and its gas accumulation mechanism. The results indicate that during the Pleistocene, sediment mainly from the southern Kunsong Uplift was transported to the Qiongdongnan Basin due to confined canyon geomorphology and developed a large channelized submarine fan in the deep-water areas. The roof of the submarine fan was cut and eroded and laterally sealed by overlying muddy channels, resulting in a dome-shaped structure, which was further covered by the Pleistocene tight mudstones forming the Ledong lithologic trap. The Yacheng coal-measure source rocks in the deep-water areas had high gas yields, which resulted in increased pressures and promoted deep gas migration into shallow reservoirs along a combined migration pathway including sand strata, structural ridges, faults, fissures, and gas chimneys. The gas accumulated in the Ledong trap as a result of the decreases in temperature and pressure. After the deep gases charged the Ledong strata, migration was blocked by the overburden consisting of Pleistocene tight mudstones, whose seal capacity was significantly enhanced by overlying gas hydrates in the deep-water areas. Meanwhile, the seawater in deep-water areas compacts the underlying cap rocks through its own weight, which enhances the seal capacity of cap rocks. In shallow reservoirs, thermogenic gases may mix with microbial gases from shallow marine source rocks, and some thermogenic gases may be reformed by microorganisms to form secondary microbial gases. The shallow Ledong gas reservoir connects the underlying gas reservoirs and the overlying gas hydrate reservoirs and vertically consists of a continuous hydrocarbon enrichment zone, which significantly expands exploration targets in the deep-water areas of the Qiongdongnan Basin. Additionally, the development model of the shallow Ledong lithologic trap and its gas accumulation mechanism may have general significance for hydrocarbon exploration in marine deep-water areas worldwide.

A Caprock Evaluation Methodology for Underground Gas Storage in a Deep Depleted Gas Reservoir: A Case Study for the X9 Lithologic Trap of Langgu Sag, Bohai Bay Basin, China

The evaluation of caprocks’ sealing capacity is exceedingly important for depleted gas reservoirs to be reconstructed into gas storage. In this paper, based on the physical sealing mechanism of caprock, four aspects of ten indexes of caprock quality evaluation were firstly selected, and the related classification standards were established. Secondly, based on the rock mechanical sealing mechanism, elastic and plastic indexes were selected to characterize the mechanical brittleness of caprock, and a brittleness evaluation method of caprock based on complete stress-strain curves was established. Then, a systematic comprehensive evaluation model (including 5 aspects and 12 evaluation indexes) for the sealing capacity of gas storage caprock was proposed, and the analytic hierarchy process (AHP) was used to determine the weight of the 12 indexes in the evaluation model, and the formula for calculating the suitability of the caprock sealing capacity was established. Finally, the geological data, laboratory, and field test data, including X-ray diffraction, poro-permeability test, displacement pressure, and tri-axial compression test, were used for the caprock sealing capacity evaluation of the X9 depleted gas reservoir, and the result from this model showed that the caprock quality is suitable for underground gas storage.

New understandings on gas accumulation and major exploration breakthroughs in subsalt Ma 4 Member of Ordovician Majiagou Formation, Ordos Basin, NW China

Geological conditions and main controlling factors of gas accumulation in subsalt Ma 4 Member of Ordovician Majiagou Formation are examined based on large amounts of drilling, logging and seismic data. The new understandings on the control of paleo-uplift over facies, reservoirs and accumulations are reached: (1) During the sedimentary period of Majiagou Formation, the central paleo-uplift divided the North China Sea in central-eastern of the basin from the Qinqi Sea at southwest margin of the basin, and controlled the deposition of the thick hummocky grain beach facies dolomite on platform margin of Ma 4 Member. Under the influence of the evolution of the central paleo-uplift, the frame of two uplifts alternate with two sags was formed in the central-eastern part of the basin, dolomite of inner-platform beach facies developed in the underwater low-uplift zones, and marl developed in the low-lying areas between uplifts. (2) From the central paleo-uplift to the east margin of the basin, the dolomite in the Ma 4 Member gradually becomes thinner and turns into limestone. The lateral sealing of the limestone sedimentary facies transition zone gives rise to a large dolomite lithological trap. (3) During the late Caledonian, the basin was uplifted as a whole, and the central paleo-uplift was exposed and denuded to various degrees; high-quality Upper Paleozoic Carboniferous—Permian coal measures source rocks deposited on the paleo-uplift in an area of 60 000 km2 providing large-scale hydrocarbon for the dolomite lithological traps in the underlying Ma 4 Member. (4) During the Indosinian—Yanshanian stage, the basin tilted westwards, and central paleo-uplift depressed into an efficient hydrocarbon supply window. The gas from the Upper Paleozoic source rock migrated through the high porosity and permeability dolomite in the central paleo-uplift to and accumulated in the updip high part; meanwhile, the subsalt marine source rock supplied gas through the Caledonian faults and micro-fractures as a significant supplementary. Under the guidance of the above new understandings, two favorable exploration areas in the Ma 4 Member in the central-eastern basin were sorted out. Two risk exploration wells were deployed, both revealed thick gas-bearing layer in Ma 4 Member, and one of them tapped high production gas flow. The study has brought a historic breakthrough in the gas exploration of subsalt Ma 4 Member of Ordovician, and opened up a new frontier of gas exploration in the Ordos Basin.

Predicting Thin Sand Beds and Subtle Lithologic Traps in the Shahejie Formation of the Chezhen Sag by Joint Investigation of Seismic Facies Using 3D Seismic Data

Predicting Thin Sand Beds and Subtle Lithologic Traps in the Shahejie Formation of the Chezhen Sag by Joint Investigation of Seismic Facies Using 3D Seismic Data

Development Model of Shallow Lithologic Traps and Natural Gas Accumulation Mechanisms in Marine Deep-Water Areas: A Case Study in the Qiongdongnan Basin, South China Sea

Development Model of Shallow Lithologic Traps and Natural Gas Accumulation Mechanisms in Marine Deep-Water Areas: A Case Study in the Qiongdongnan Basin, South China Sea

Deep-water gravity flow deposits in a lacustrine rift basin and their oil and gas geological significance in eastern China

The types, evolution processes, formation mechanisms, and depositional models of deep-water gravity flow deposits in a lacustrine rift basin are studied through core observation and systematic analysis. Massive transport of slide and slump, fluid transport of debris flow and turbidity currents are driven by gravity in deep-water lacustrine environment. The transformation between debris flow and turbidity current, and the transformation of turbidity current between supercritical and subcritical conditions are the main dynamic mechanisms of gravity flow deposits in a lake basin. The erosion of supercritical turbidity current controls the formation of gravity-flow channel. Debris flow deposition gives rise to tongue shape lobe rather than channel. Deep-water gravity flow deposits are of two origins, intrabasinal and extrabasinal. Intrabasinal gravity flow deposits occur as single tongue-shape lobe or fan of stacking multiple lobes. Extrabasinal gravity-flow deposits occur as sublacustrine fan with channel or single channel sand body. However, the nearshore subaqueous fan is characterized by fan of stacking multiple tongue shape lobes without channel. The differential diagenesis caused by differentiation in the nearshore subaqueous fan facies belt results in the formation of diagenetic trap. The extrabasinal gravity flow deposits are one of the important reasons for the abundant deep-water sand bodies in a lake basin. Slide mass-transport deposits form a very important type of lithologic trap near the delta front often ignored. The fine-grained sediment caused by flow transformation is the potential “sweet spot” of shale oil and gas.

New understandings and potential of Sinian–Lower Paleozoic natural gas exploration in the central Sichuan paleo-uplift, Sichuan Basin

Since the Anyue Gasfield, located in the central Sichuan paleo-uplift of the Sichuan Basin, was discovered, great efforts have been made to work on natural gas exploration and discovery in the Sinian–Lower Paleozoic in the north slope of present paleo-uplift which has similar depositional settings. It is verified by the breakthrough of natural gas exploration in the second Member of Upper Sinian Dengying Formation in the north slope of central Sichuan paleo-uplift by wildcat well PT1 and the new sign of natural gas exploration in the Canglangpu Formation of Lower Cambrian and the fourth Member of Dengying Formation by Well JT1 that there are also favorable conditions for the formation of large-scale gas province in the north slope. In order to determine the natural gas exploration potential of Sinian–Lower Paleozoic in the central Sichuan paleo-uplift and provide the guidance for the following exploration deployment, this paper analyzed the petroleum geological conditions of Sinian–Lower Paleozoic in the north slope. And the following research results were obtained. First, the marginal platform belts in the second and the fourth Member of Sinian Dengying Formation in the north slope are basically separated areally, and they are superior to the Gaoshiti–Moxi area in terms of marginal platform width and sedimentary thickness and are intrinsically advantageous in sedimentation. Second, compared with the Gaoshiti–Moxi area, the reservoirs of Sinian Dengying Formation in the north slope are better in reservoir conditions, and many sets of quality reservoirs are developed vertically in Sinian–Cambrian. Third, hydrocarbon accumulation elements of Sinian Dengying Formation are better allocated in the north slope. Lithological traps are developed with a larger cumulative area. Wells JT1 and PT1 verify that there is gas in the lithological trap of the fourth and the second Member of Dengying Formation and large-scale lithological gas reservoirs are developed in the slope setting. In conclusion, compared with the Gaoshiti–Moxi area, the Sinian–Lower Paleozoic in the north slope is superior in petroleum geological conditions and has the advantage of multi-layer stereoscopic exploration vertically, presenting a great natural gas exploration potential and promising exploration prospects, so it is a new important strategic zone of conventional natural gas exploration in the Sichuan Basin.

Recovering the phase diagram of condensate gas reservoir in Well TZ86, Central Tarim Basin using PVTsim with geochemical inputs

The phase diagram of the condensate gas reservoir in the Ordovician lithologic trap of Well TZ86 was recovered using PVTsim software with geochemical inputs (well fluid components and experiment data). The results show the error between the recovery and measured phase diagrams is within a reasonable range, indicating the reliability of the recovery result. The recovery phase diagram shows an order of CP-Pm-Tm, and the current reservoir temperature and pressure (138.49°C, 65.10MPa) confirms that the fluid belongs to the condensate gas phase currently and turns to the condensate gas-condensate oil coexistence phase when it is mined to the surface condition. The components rich in gaseous hydrocarbons caused by the gas charging in the Late Himalayan and the temperature and pressure condition determined the formation of condensate gas reservoirs in the Tazhong Uplift.

Identification and characterization of complicated lithological traps within sequence stratigraphic framework – A case study in Yacheng uplift, Qiongdongnan Basin

With the increasing of lithologic reservoir reserve ratio, lithologic traps are playing an increasingly important role in worldwide oil and gas exploration. Focusing on the lithological traps in the Yacheng Uplift area, Qiongdongnan Basin, this paper introduces a work flow for identification and characterization of lithological traps within sequence stratigraphic framework. The first stepis to divide the Miocene Meishan and Sanya formation in the Yacheng Uplift area into 2 third order sequences and 5 fourth order sequences (SQ1-SQ5), and to indentify the type, characteristics and distribution of lithological traps, thus confirming the main type of lithological trap is muddy channel lateral-sealing traps which are concentratedly distributed in the highstand system tract (SQ3) of Sanya formation. The second step is to quantitatively characterize the shape, channel lateral-sealing effectiveness and oil-gas-bearing possibility of the traps by some geophysical methods such as seismic attributes, seismic slice, pre-stack and post stack acoustic inversion. By using the above techniques, a lot of channel lateral-sealing lithologic traps have been found and accurately evaluated, thus putting forward the new oil and gas exploration direction in the study area.

Accumulation and exploration of petroleum reservoirs in west slope of northern Songliao Basin, China

Abstract On the basis of the present situation of oil and gas exploration and geological research of the west slope in the northern Songliao Basin, the factors controlling reservoir formation, oil and gas migration and accumulation, have been re-examined from the aspects of structure, deposition and reservoir formation. The results show that: (1) The west slope is a gentle slope which overlaps to the west, and nose structure is developed near the hydrocarbon generation depression, which is in the dominant direction area of hydrocarbon migration. A series of NE structural belts are developed on the slope and are favorable places for oil and gas accumulation. (2) The west slope can be further divided into the upper slope and the lower slope, and there are many kinds of oil and gas reservoirs, including structural, structural-lithologic and lithologic ones. In the upper slope, the major oil layer is Sartu controlled by structure; in the lower slope, multi-layers are oil-bearing, and the oil reservoirs are mostly composite ones. (3) Faults, unconformity surfaces and continuous sand bodies are the main channels of oil and gas migration; structure, sand body and fault jointly control the oil and gas enrichment in the slope; and the matching relationship between micro-amplitude and sand body, small fault and sand body control the oil and gas accumulation. On the basis of the above research, fine identification and effectiveness evaluation technology of composite trap has been developed through extensive study. Combination traps were identified by multiple technologies, including fault classification, micro-amplitude structure identification, fine sedimentation research, and lithologic trap identification by waveform indication inversion; and then the configuration relationship between fault and sand body, structural amplitude and sand body were analyzed to set up the evaluation criteria of effective traps. According to the criteria, the traps were selected to enhance the exploration success rate.

COMPUTER MODELING OF TERRIGENOUS SEDIMENTATION AS A NEW TOOL FOR PREDICTION OF HYDROCARBON RESERVOIR ARCHITECTURE

The subject of the article is a new approach based on the integration of seismic facies analysis with modeling of sedimentation system – submarine fan, which is a potential lithological trap. The new approach allows introducing sedimentation systems modeling into the traditional process of seismic data interpretation. The approach was tested on data of the target interval in Cherkashin formation (West Siberia, Priobskoye-Salym region, Neocomian progradation complex). The presence of submarine fans selected as test-subjects for modeling is confirmed by lithofacial analysis, distribution of effective thicknesses over the area according to drilling results, seismic facies analysis. Two criteria were used to evaluate the simulation results: compare the shape of the model with the results of seismic facies analysis and comparison of the parameters describing the geological process and selected in the modeling process with modern data on turbidite systems. Modeling of submarine fan sedimentation was carried out in the Geological Process Modeling (GPM) module of the Petrel software (Schlumberger), Seismic Facies Analysis – in tratimagic software (Emerson). The approach can be recommended for studying of deep-water Achimov deposits; predicting the development of the distal parts of the submarine fan, not evident in the seismic data and predicting the internal architecture of the potential lithological traps.

Sequence Control Factors and Hydrocarbon Accumulation Pattern of Es3 in Kenxi Slope Zone of Bohai Bay Basin

Xu, X.-Y.; Wang, W.-F.; Li, X.-S., and Chen, H., 2019. Sequence control factors and hydrocarbon accumulation pattern of Es3 in Kenxi slope zone of Bohai Bay basin. In: Gong, D.; Zhu, H., and Liu, R. (eds.), Selected Topics in Coastal Research: Engineering, Industry, Economy, and Sustainable Development. Journal of Coastal Research, Special Issue No. 94, pp. 91–95. Coconut Creek (Florida), ISSN 0749-0208.Based on outcrops, rock cores, seismic, well logging, and other data, the study traces the evolutions of sequence stratigraphy, the types of sedimentary facies and sequence development control factors of Es3 in the Kenxi slope zone of Bohai Bay basin. The findings show that the stratum in this area can be divided into three third-order sequences, eleven fourth-order sequences and seven system tracts; the types of sedimentary facies include fan delta, sublacustrine fan, semi-deep lake, and deep lakes, etc.; sedimentary facies belt, lithologic trap patterns, hydrocarbon migration and preservation conditions are the core factors controlling lithologic reservoirs in this area. The exploration prospect in this area have been predicted. It is believed that there is a large exploration potential in the upper Es3 sub-member, the south of well K25-1 of the west Kenxi prominence.

High Resolution Sequence Stratigraphic Division and Isochronous Framework Establishment in Wuerxun-baer Sag

High-resolution sequence stratigraphy and related technical methods have been widely used in the analysis of complex and changeable continental petroliferous basins in China in recent years. At present, the main fields of oil and gas exploration in Hailaer Basin have been transferred to lithologic reservoirs, so it is very important to find lithologic traps and analyze the favorable formation conditions of lithologic traps. The tectonic evolution, sequence stratigraphic framework and tectonic control over the sedimentary system of Nantun Formation have been studied in depth by predecessors. However, the division accuracy of Nantun Formation can only reach the level of system tract, and the distribution characteristics of sand bodies and source-reservoir allocation under high-resolution sequence framework are seldom studied. Therefore, on the basis of predecessors’research, the author synthetically uses the related technology and methods of high-resolution sequence stratigraphy, and according to the sequence boundary characteristics reflected by drilling, core, seismic and logging data, establishes the high-resolution sequence stratigraphic framework through fine division and correlation of base-level cycles at various levels. The distribution law of sand body under its control and the source-reservoir allocation relationship favorable to lithologic trap formation are studied in order to have important guiding significance for further tapping the potential of lithologic reservoir.

Characteristics of faults and their significance in controlling the oil accumulations in Honghe field, Ordos Basin

The Chang 8 (C8) reservoir is a transitional oil pool type from a tight oil sandstone to a lithologic trap in the Upper Triassic Yanchang Formation of the Honghe oil field, southwest Ordos Basin, China. Vertical faults play important roles in the formation and distribution of the sweetness. To predict favorable high-yield well areas and guide horizontal well deployment, 3D seismic data are used to analyze these faults’ distribution, formation phase, stress property, and control on the oil accumulation. The results indicate that the X shape of the east–northeast and north–northeast conjugate shear faults were formed during the Indosinian epoch, and the northwest faults were formed during the Yanshan epoch. Meanwhile, the earlier east–northeast faults partially experienced reactivity and rework later but the north–northeast faults ceased since the Indosinian. The strike-slip faults in the area are classified into three categories based on their intensity, density, formation stage, and oil control effectives: types I, II, and III. The type I faults extended in the northwest and east–northeast directions and were cut from the Ordovician up to the top Cretaceous. They are characterized by the maximum extensions aerially and vertically thereby faulting the thickest strata. Type II faults mainly extended in the east–northeast direction and were cut from the Triassic up to the top Cretaceous in small intension. Type III faults mainly extended in the north–northeast direction and were cut from the Ordovician up to the Upper Permian. The faults connect the C8 sandstone reservoir and the Chang 7 source shale by penetrating the interlayered silty mudrock in between them. The low permeable reservoir also requires additional permeability imparted by the type I faults. High-yield horizontal wells are distributed along intensive type I faults. The water-bearing zones or oily water zones are mostly related to no fault or to the type II and III faults.

Sedimentary Characteristics and Lithological Trap Identification of Distant Braided Delta Deposits: A Case on Upper Cretaceous Yogou Formation of Termit Basin, Niger

Lithological trap identification in thin sand and thick shale layers is still a challenge for hydrocarbon exploration. Based on the high-resolution sequence stratigraphy theory and the establishment of high resolution sequence stratigraphy framework with seismic-well tie, the dynamic deposition process of braided river delta sands on late Cretaceous Yogou formation has been analyzed on 62 wells in passive rift Termit basin with multi-stages depressions and reversals. (1) Six kinds of sedimentary microfacies and three major reservoir sands with multi-stages stacking and lateral migration are in Yogou formation; (2) Based on Accommodation space/Sediments supply change and the deposition progress, sedimentary facies distribution in each member of YS3 sub-formation has been done according to sands thickness statistics of sedimentary micro-facies, narrow-time seismic attributes and slices analysis, multi-sources braided river delta depositional model has been concluded; (3) Based on source rock and caprock evaluation, with reservoir sands distribution and faults impact on Yogou formation of Termit basin, four types of traps, including structure-lithology, Structure-strata, stratigraphic and lithology are concluded. Traps influencing factors, i.e., structure geometry, sands distribution, paleotopography, stratigraphy cycling, sand/shale lateral connection, reservoir quality and so on, have different impacts on these traps, and different lithologic-stratigraphy traps have different exploration risks. Structure geometry and sands distribution are very important for the structure-lithology traps; structure geometry and paleotopography are the key factors in Structure-strata traps. Sands distribution and reservoir quality can be focused on lithology traps. Moreover, paleotopography and sand/shale lateral connection are significant on stratigraphic traps. Therefore, different hydrocarbon accumulation types of lithological traps have been established.

Sedimentary Characteristics and Lithological Trap Identification of Distant Braided Delta Deposits: A Case on Upper Cretaceous Yogou Formation of Termit Basin, Niger

Lithological trap identification in thin sand and thick shale layers is still a challenge for hydrocarbon exploration. Based on the high-resolution sequence stratigraphy theory and the establishment of high-resolution sequence stratigraphy framework with seismic-well tie, the dynamic deposition process of braided delta sands on late Cretaceous Yogou formation has been analyzed on 62 wells in passive rifting Termit basin with multi-stages depressions and reversals. (1) Six kinds of sedimentary microfacies and three major reservoir sands with multi-stages vertical stacking and lateral migration are in late Cretaceous; (2) Based on accommodation space/Sediments supply change and depositional progress, sedimentary facies distribution evolution has been done according to sands thickness statistics of sedimentary micro-facies, and narrow-time seismic attributes and slices analysis, multi-sources braided delta depositional model has been concluded; (3) Based on source rock and caprock evaluation, with reservoir sands distribution and faults impact on Yogou formation of Termit basin, four types of traps, including structure-lithology, Structure-strata, stratigraphic and lithology are concluded. Traps influencing factors, i.e., structure geometry, sands distribution, paleotopography, stratigraphy cycling, sand/shale lateral connection, reservoir quality and so on, have different impacts on these traps, and different lithologic-stratigraphy traps have different exploration risks. Structure geometry and sands distribution are very important for the structure-lithology traps; structure geometry and paleotopography are the key factors in Structure-strata traps. Sands distribution and reservoir quality can be focused on lithology traps. Moreover, paleotopography and sand/shale lateral connection are significant on stratigraphic traps. Therefore, different hydrocarbon accumulation types of lithological traps have been established.

Condensate origin and hydrocarbon accumulation mechanism of the deepwater giant gas field in western South China Sea: A case study of Lingshui 17-2 gas field in Qiongdongnan Basin

Abstract Based on the geochemical characteristics of condensates and gases, in combination with geological background, the origin and formation of condensate and oil-gas accumulation mechanism of the deepwater Lingshui 17-2 gas field in the Qiongdongnan Basin, western South China Sea are discussed. The condensate from Lingshui 17-2 gas field has the features of low density, low wax content and high Pr/Ph value. The condensate is generated at mature stage while the co-existing gas is dominated by high mature gas. The oil and gas are derived from the Oligocene Yacheng Formation source rocks rich in terrestrial organic matter. The formation of the condensate is related not only to the source rock, but also to the later “gas-washing”. The light oil and gas which was generated at oil-window stage and accumulated in reservoirs have strong reaction of evaporation fractionation during “gas-washing” by high mature natural gas injected largely at later period. The Yacheng source rocks provided sufficient gas and some light oil, diapiric fractures formed migration paths for oil and gas, and two stages of hydrocarbons charged and accumulated. The Miocene Huangliu Formation turbidite sandstones lithologic trap and Paleogene structural trap close to source Kitchen in the central canyon have favorable oil-gas exploration potential.

The potential for locating hydrocarbon traps in the Rotliegend formation, based on results of experimental seismic surveys

Within the framework of the Operational Programme Innovative Economy 2007-2013, studies were carried out for improving the effectiveness of using seismic surveys in the prospecting and exploration of gas fields in the Rotliegend formation. The completed experimental seismic surveys provided new, very interesting data on the geological structure of the deeper part of the Rotliegend basin (>4000 m), which is practically very important for prospecting for new gas fields. The interesting results were obtained along virtually the entire cross-section, particularly in two segments: the southern segment – in front of and within the Czlopa-Szamotuly tectonic zone (the Szczecin-Gorzow segment) and the northern segment – in the area of the Zabartowo-1 and Zabartowo-2 wells (the Pomeranian segment). In the northern segment, for the first time in the history of studies of the Lower Permian Basin, it was possible to record reflections from the horizon of the aeolian sandstones occurring within the playa silty-clayey deposits (543 m below the top of the Rotliegend formation), which represents a significant achievement from the point of view of prospecting for hydrocarbon resources. This horizon, characterized by favourable reservoir properties, pinches out northward. It is a very important piece of information because systems of lithologic traps may be related to such pinchouts. In the central part of the Obrzycko-Zabartowo profile, in the Chodziez area, a vast structural elevation occurs, approximately 10–12 km wide and with the amplitude of 250–300 m, which may be a potential large structural or structural and lithologic trap. These newly discovered zones of occurrence of gas traps in the seismic profiles made by the AGH University of Science and Technology within the framework of the research project, will be implemented in a detailed seismic exploration during the exploratory work of Polish Oil and Gas Company (PGNiG SA).