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Abstract
Based on analysis of fault activity, linked well survey line along fault strike, fault internal structure and its matching relation with sand-body, and specific reservoir distribution of Tangzhuang-Xiaozhuang area in Huimin depression, hydrocarbon migration along fault strike was studied and the migration pathway was confirmed by geochemistry data. The results show that fault is a complex three-dimensional geological body with certain volume and heterogeneous structures. It is an important gateway for hydrocarbon migration, and there are migration components in vertical, lateral and strike of fault. Hydrocarbon migration styles along fault strike could be divided into migration through transporting ridge in fault and migration through transporting ridge composed by sand-body and fault. In different periods of fault activity, different transporting styles dominate in hydrocarbon migration along fault strike with gradual transition. In fault activity, early stationary and short-term quiescent period, hydrocarbon migration is mainly through transporting ridge in fault, and it is mainly through transporting ridge composed by sand-body and fault in long-term quiescent.
... Regarding the controlling effect of oil-source faults on hydrocarbon accumulation and distribution, there have been many studies on faults transporting hydrocarbons (Aydin, 2000;Fu and Wang, 2019;Cong et al., 2020;Liang et al., 2022), fault zone structures (Allan, 1989;Bense, et al., 2003;, fault lateral sealing ability (Chen et al., 2010;Fossen and Rotevatn, 2016;Lyu et al., 2016), source-fault configurations , faultsand configurations Zhou et al., 2019;Luo et al., 2020), fault-caprock configurations (Kumar et al., 2019;Shi et al., 2019;Wang et al., 2021), the development degree of sand bodies (Luo et al., 2012;Fu and Yu, 2021), fault trap characteristics (Jackson et al., 2006;Hu and Lyu, 2019;Song et al., 2020), etc., many studies have confirmed that the hydrocarbon accumulation position near an oil-source fault is related to the transportation capacity of the oil source fault (Lampe et al., 2012;Jiang et al., 2017;Liu et al., 2017;Fu and Wang, 2018). Not all parts of an oil-source fault can vertically migrate hydrocarbon and there are dominant migration conduits. ...
Oil-source faults play an important role in controlling hydrocarbon accumulation, and traps near these faults are often the most favorable positions for hydrocarbon accumulation. However, not all nearby traps can accumulate oil and gas. Therefore, it is necessary to predict favorable positions of hydrocarbon accumulation controlled by oil-source faults. Based on the analysis of hydrocarbon accumulation patterns near oil-source faults, a new method coupling hydrocarbon transportation positions of oil-source faults and favorable traps to predict favorable hydrocarbon accumulation positions is proposed. In this method, the hydrocarbon transportation positions are identified by the paleo activity rate during the hydrocarbon accumulation period; the favorable traps are identified by superimposing the position of fault traps, the lateral sealing position of faults, and the distribution of sand bodies with the ability to store oil and gas. Finally, the sites of overlap between the hydrocarbon transportation positions and the favorable traps are regarded as favorable positions for hydrocarbon accumulation under the control of oil-source faults. This method is applied to predict the favorable positions of hydrocarbon accumulation in the Ban 2 oil group of the Shahejie Formation near the Dazhangtuo fault, in the Qikou Sag (Bohai Bay Basin, East China). The results showed that the favorable positions of oil and gas accumulation along the Dazhangtuo fault in the Ban 2 oil group are mainly distributed in the northeast and center of the fault, and match well with the locations of the discovered oil and gas. Therefore, this method is useful to predict favorable hydrocarbon accumulation positions controlled by oil-source faults.
The oblique transfer zone in the Fushan Sag, a syndepositional dome sandwiched between the Bailian and Huangtong sub-sags, has been the most important exploration target. The major oil observation occurs in the E2l
1L+M and the E2l
3U. 46 oil and rock samples reveal that the oil in the transfer zone is mostly contributed by the Bailian sub-sag, though the source rock conditions, hydrocarbon generation and expulsion histories of the Bailian and Huangtong sub-sags are similar. The E2l
3U oil, characterized by high maturity, Pr/Ph ratio and oleanane/C30-hopane ratio, shows a close genetic affinity with the E2l
3b source rocks, while the E2l
1L+M oil, characterized by lower maturity, Pr/Ph ratio and oleanane/C30-hopane ratio, is suggested to be derived from the E2l
1+2b source rocks. The homogenization temperatures of aqueous fluid inclusions, taking the burial history of the reservoirs into account, reflect that the oil charge mainly occurred from mid-Miocene to Pliocene in the oblique transfer zone. The oil transporting passages include connected sand bodies, unconformities and faults in the Fushan Sag. Of these, the faults are the most complicated and significant. The faults differ sharply in the west area, the east area and the oblique transfer zone, resulting in different influence on the oil migration and accumulation. During the main hydrocarbon charge stage, the faults in the west area are characterized by bad vertical sealing and spatially dense distribution. As a result, the oil generated by the Huangtong source rocks is mostly lost along the faults during the vertical migration in the west area. This can be the mechanism proposed to explain the little contribution of the Huangtong source rocks to the oil in the oblique transfer zone. Eventually, an oil migration and accumulation model is built in the oblique transfer zone, which may provide theoretical and practical guides for the oil exploration.
The controlling effect of fault dense belts on the preferred direction of oil-gas migration is discussed and favorable parts of fault dense belts for oil accumulation inside and outside oil source area are discussed based on the characteristics of fault dense belts of Fuyang Formation in Sanzhao Depression, combining with the oil and gas distribution of in Fuyang Formation of study area in this paper. The results show that there are four types of fault dense belts in Fuyang Formation of Sanzhao Depression, namely antithetic-graben-antithetic fault terrace, horst-graben-antithetic fault terrace, antithetic-graben-consequent fault terrace, and horst-graben-consequent fault terrace. The strike of fault dense belt is the preferred direction of oil-gas migration and horsts and antithetic inside oil source area of fault dense belts are favorable parts for oil accumulation where the strikes of fault dense belt and the layer are parallel or small-angle intersection. Inside oil source area horst and antithetic of fault dense belts are favorable parts for oil accumulation. And horsts and antithetic outside oil source area are favorable parts for oil accumulation and the grabens are secondary favorable ones where the angle of the strikes of fault dense belt and layer ranges from 0 to 45 degrees. The antithetic and horsts are preferential for oil accumulation where the angle ranges from 45 to 90 degrees.
Based on a comprehensive analysis of the generation characteristics of source rocks as well as the difference and the quantitative evaluation of fault activity, the authors studied the relationship between the difference of fault activity and the Neogene hydrocarbon enrichment during the hydrocarbon generation period in Qikou Sag of Bohai Bay Basin. The results show that the fault activity differences during the main hydrocarbon generating periods from Neogene to the present can be divided into five categories, i.e., the early-acted-late-declined type, the gradually-acted-turned declined type, the gradually-declined-turned acted type, the early declined-late acted type and the acted stable type. The relationship between the difference of fault activity and the Neogene hydrocarbon enrichment can be revealed by the fault activity difference coefficient (FDC). According to the fault activity rate in the phase of the Minghuazhen Formation and the fault activity difference coefficient,the faults are divided into 3 types, namely I , II and III. Hydrocarbon is mainly concentrated in the favorable traps controlled by type I and type II faults whose activity rate and FDC are relatively high. The enrichment of oil and gas is poor in the traps controlled by type III faults whose activity rate and FDC are relatively low. The scale of oil and gas migration is controlled by fault activity rate, while the extent of hydrocarbon enrichment is controlled by FDC.
Based on the data of seismic, well logging, mud logging and analysis assay, the control of hydrocarbon migration and accumulation by fault and sand body in the Neogene of northern coastal area, Shengli Oilfield was analyzed. The results show that fault activity controls the size and layers of oil and gas distribution. Fault sealing controls the maximum oil height of reservoir, and the sealing ability of fault-screened trap is better than that of co-dip fault-screened trap. The sand groups whose sand to formation thickness ratios are less than 0.15 are not benefical to hydrocarbon migration and accumulation, the sand groups whose sand to formation thickness ratios are 0.15-0.5 are considered favorable to hydrocarbon accumulation and the sand groups whose sand to formation thickness ratios are greater than 0.5 are beneficial to hydrocarbon migration. The combination of trunk fault and main channel sand body is the most favorable combination for hydrocarbon migration and accumulation. The petroleum exploration in non-source rocks layers can be effectively guided by the exploration thinking way of "fix district-fix sand group-fix point".
As geological investigation of the Wulungu depression in the Northeast Junggar Basin goes on in recent years, rich oil and gas resources are founded in the depression. But compared with the other part of the basin, the investigative level of oil and gas in the Wulungu depression is still low. This article systematically explores the law of oil and gas accumulation based on the structural characteristics and fault evolution. The Wulungu depression developed three sets of source rocks. The Dishuiquan Group and Batamayineishan group of Carboniferous dark mudstone is the most important source rock. According to the structural characteristics and evolution, the piedmont of Kelameili Mountain is the favorable oil and gas accumulation areas. The Wulungu depression is the oil-gas accumulation zone controlled by thrust faults, the oil and gas reservoirs in this area are characterized by zonal distribution along the NW-striking faults. The active deep-seated faults are the major hydrocarbon migration paths; the shallow faults mostly distributed in the Kalasayi fault, may damaged the early oil and gas reservoirs but benefit to the formation of secondary reservoirs in some degree. As the tectonic migration from north to south, the fault intensely active period matched well with the hydrocarbon generation. Structural traps in the root zone in the north developed from Late Indocinian stage to Early Yanshan stage, the main hydrocarbon reservoir types are faulted noses and faulted blocks related to the thrusting and secondary lithofacies reservoirs. Structural traps in the front zone in the south formed at Middle-Late Yanshan stage, the main hydrocarbon reservoir types are fault- related bends, fault-propagation folds, angular unconformity reservoirs and lithofacies reservoirs, but most of them are damaged by normal faulting during Himalayan period.
A fault is not a simple plane, but a belt composed by a series of breaking planes or secondary faults, which is contradict with the hypothesis of the theory of cross-fault lithology juxtaposition. According to the characteristics of faults, the relation of lithology juxtaposition was discussed by two types of fault plane and fault belt, furthermore, twelve models were divided according to the cross-fault lithology, displacement pressure and fault throw. The researched relation of lithology juxtaposition of Ying 32 fault shows that lithology juxtaposition is diversity in different sand groups, and the lithology juxtaposition of sand groups of 1 is the best one and sand groups of 8 is the worst one, which induce different sealing degree of faults. Along the strike of Ying 32 fault, fault throw is gradually small from north to south, the probability of cross-fault lithology juxtaposition increases for the same reservoirs and lithology juxtaposition becomes poor. The remaining oil tapping of both sides of Ying 32 fault should be particular emphasis on the north of fault.
Based on fault, sand body and matching relationship of sand-fault, geological, seismic and geochemical data were used to study the oil resource, effective migration pathways and pool-forming model of Xindong area in Dongying depression. The results show that hydrocarbons of Xindong area mainly come from Es4 source rock, petroleum bearing in structural flanks originates from adjacent sag. Niuzhuang sag, Minfeng sag and Xindong underlying source rock offer hydrocarbon for structural core where the origin of petroleum is complicate. According to the pattern of nitrogen compounds, hydrocarbons of Xindong area come from northwest Minfeng sag and southwest Niuzhuang sag. The effective migration pathways of Xindong area are dominated by the X172, X120 and Y20 faults. So the pool-forming models are generalized to three types; sand control pool-forming, trap control petroleum accumulation, the matching relationship of sand-fault control pool-forming, which are used to guide the exploration survey of Xindong area.
The development of overpressure in sedimentary basin has the important material and energy effects on oil and gas accumulation. Overpressure retardation of organic matter maturation can keep the deeply buried source rocks and the relatively old source rocks in the favorite hydrocarbon-generation and hydrocarbon-expulsion stage simultaneously with the young source rocks. The resultant delay in fluid release would lead to the centralized expulsion of hydrocarbon generated at different temperature and maturity, which increased the interval and volume of effective source rocks and further increased the accumulative hydrocarbon-expulsion rates. Such a process is favorable for late-stage rapid accumulation of hydrocarbon. Overpressure can cause natural hydraulic fracturing of strata, which is an important way for the primary expulsion of overpressure fluid flow. The secondary expulsion of overpressure fluid was controlled by overpressure development and the nature of faults. This kind of fluid flow may be called as overpressure-fault controlled fluid flow. The combination of the material and energy effects of overpressure determined the rapid hydrocarbon accumulation controlled by neotectonic movement or late-stage tectonic activities. The hydrocarbon accumulation was accomplished by episodic fluid injections. Each episode included steady converging and transient injection periods. The episodic accumulation rate was very high, and the large oil-gas fields could be formed within 0.1 million years. The tectonic activities had the constructive role for hydrocarbon accumulation. The neotectonic movement or late-stage tectonics controlled hydrocarbon distribution.
There is an internal structural discrepancy between plastic faults and fragile faults: a fragile fault zone consists of a crushed zone and an induced fracture zone characterized by fault rock and associated fractures; a plastic fault zone consists of several big fractures filling the fault gouge without an induced fracture zone. Associated fractures inside a crushed zone, a fault rock zone without adhesive power and induced fracture zones may all allow petroleum migration. So a fragile fault is vertically sealed only when all three migration pathways are sealed. When the associated fractures are sealed, the plastic fault is also sealed. Based on the seal mechanism, the sealing conditions of the three migration pathways are analyzed in this paper: the seal on a fault rock zone without adhesive power is dependant on the content of fault mud; the seal of an associated fracture inside a crushed zone is dependant on the relationship between the section pressure and fragile strength of the fault gouge; the seal quality of an induced fracture zone is controlled by the quantity of later rock-forming fill. This paper presents a method for evaluating the vertical seal of disparate faults using the section pressure, content and plastic strength of fault gouge and the quantity of later rock-forming fill. For example, the vertical seal of the F1 fault of Kela 2 structure in Kuqa depression was evaluated according to this method. The results indicate that vertical seal on the F1 fault had the following properties: section (1) and section (3) show fragility, but section (1) is not sealed, as a result of an induced fracture that was not filled; section (3) is sealed; section (2) is a plastic fault with a good vertical seal. This is one of tin key reasons that natural gas gathered on a large scale and formed a field in Kela 2 structure.
Based on study on the formation conditions of overthrust and its seal mechanism, this paper considered that the main factors influencing vertical seal of overthrust depended on deformation characteristics of plastic rocks accompaning overthrust, pressure exerted on fault zone and mudstone content in fault zone. A simulation experiment on the quantitative relation between mudstone content in fault zone and vertical seal of overthrust was carried out. In addition, this paper analyzed the influence of deformation characteristics of rocks on vertical seal of fault. The evaluation standard for vertical seal of fault was established which put forward a method evaluating vertical seal of fault by the mudstone content in fault zone, the pressure exerted on fault plane and the deformation characteristics of gypsum-bearing mudstone. Taking the Kela 3 Structure in the Kuqa Depression as an example, the vertical seal of fault was studied, and the results indicated the method being reliable.
Oil-gas reservoirs in middle and shallow strata exist in the traps of lower walls of faults in the northern edge of Qaidam Basin, which is opposite to the Kunbei area in the southern edge of the basin. Geological conditions of faults and faulted belts in the middle-shallow strata show that the reasons for the enrichment of oil and gas in the lower walls of detachment faults in the northern edge are as follows: detachment faults in the middle-shallow strata don't incise source rocks and cannot provide oil and gas to the hanging wall; faulted belts have zonations because of sealing differences in the direction vertical to fault-plane; the sealing of the zones below the main fault-plane gets better gradually down the slope, providing favorable conditions for the formation of multilayer oil-gas reservoirs in the lower walls of detachment faults. The lower walls of detachment faults connected by faults incising source rocks are important exploration fields in the northern edge of the basin.
Studies of fracture networks and hydrothermal mineralisation of several faults affecting Jurassic siliceous sedimentary rocks, exposed in the Rapolano geothermal area (hinterland of the Northern Apennines, Italy), allow us to characterise the fault zone architectures and their perme-ability features. The study structures are normal faults with displacements of anything up to 30e40 m, belonging to a fossil hydrothermal sys-tem, Pleistocene in age. The fault zones are characterised by asymmetrical damage zones which are thickest in the hangingwall blocks. Fracture networks mainly consist of a subvertical fracture set at about 45 with respect to the fault plane. Widespread hydrothermal alteration (illite/smec-tite and kaolinite) and mineralisation consisting of quartz, calcite, dolomite, malachite, azurite and iron oxides characterise the fractures of the damaged rocks. This mineralisation suggests the occurrence of extraformational fluid circulation during the latest stage of faulting. Fault cores are characterised by cemented fault rock up to 25 cm thick, consisting of protocataclasite and ultracataclasite layers, grading to crush and fine crush breccia strongly affected by minor C 1 -type shear planes. Fault cores represented barriers to fluid flow during the latest stage of faulting, whereas they acted as conduits during the initial stages. Fault zones with similar features are presently affected by hydrothermal circulation (thermal water up to 39 C, and CO 2). The hydrothermal fluids give rise to several thermal springs and are exploited at depth for thermal resorts and CO 2 extraction, both in the Rapolano area and surroundings. A similar scenario characterises the Larderello-Travale and Mt. Amiata geo-thermal areas, where hydrothermal fluids and steam are industrially exploited for electricity production. The main results of this study are that the damage zone is asymmetrical and widest in the hangingwall. This could represent a useful contribution to the prediction of hydrothermal fluid pathways and geothermal targets in all the geothermal areas with similar features to those described here.
Fault zone structure and permeability data from the Median Tectonic Line (MTL) in Mie Prefecture, Southwest Japan suggest that fault permeability models are currently too simplistic for such large structurally complex fault zones. Ryoke Belt mylonites are cut by mineralised brittle structures up to 300 m North of the MTL that show evidence of fluid circulation. The Sambagawa schist on the south side of the MTL is deformed into foliated quartz/phyllosilicate gouge across a 15-m-wide zone. The complex fault contact area has foliated cataclasite up to 4 m wide, and is cut by a narrow central planar slip zone that probably represents the most recent seismogenic principal displacement zone. Laboratory-determined permeability data show wide variation with fault rock microstructure (e.g. gouge microclast size), controlled by structural position in the fault zone and slip zone intersections. Central slip zone gouges have the lowest permeabilities of all of the fault rocks studied. Fault permeability models should take into account asymmetry where widely contrasting protolith lithologies exist and large permeability variations within a complex central fault zone ‘core’. Pore pressure evolution during rupture propagation may vary greatly because of this complexity, but thermal pressurisation is feasible above certain pressures if the slip remains within fine-grained gouge. The different deformation behaviours of contrasting protolith lithologies control the fault zone fabrics and hence final permeability structure.