Steep Slope Belt Research Articles

A Method for Defining Sedimentary Characteristics and Distributions and Its Application in Qinnan Depression, Bohai Bay Basin

A new method incorporating geophysical analysis and geological analysis is proposed to define the sedimentary characteristics and distributions in basins with few drilling wells to promote the exploration of reservoirs. This method is applied to a study, through which its principles, closed-loop workflow and technologies are introduced in detail and the sedimentary characteristics and distributions of the study area are accurately defined. During the application process of the method, a compatible geological model is established, based on which the seismic data are interpreted and the results derived from the interpretation are further verified via seismic forward modeling. The study results exhibit a successive sand-rich deposition from the retrogradational gully-filling gravity flow deposition including near-shore fans, slope fans and basin-floor fans delimited by different slope break belts in transgressive sequences to the progradational delta deposition in a retrogressive sequence including braided river deltas with a long extension distance and fan deltas developed along a steep slope belt. And the potential reservoirs are located at the point-out sites of sand bodies with lower average P-wave velocities than those of muddy sediments. The proposition and application of this method are of great significance for oil and gas exploration.

Gravity flow sedimentary characteristics and sedimentary model of the 1st member of the Tengger formation in Saihantala depression (Erlian Basin, China)

Based on core, well-logging and seismic data, the gravity flow characteristics and sedimentary model of the 1st member of the Tengger formation in the eastern steep slope belt of the Sai-4 tectonic belt in the Saihantala depression (Erlian Basin, China) have been systematically studied; further, the sublacustrine fan sedimentary model in the study area has been established. The main types of gravity flow identified in the study area are sandy debris flow deposits and turbidity flow deposition. Affected by paleo-geomorphology, provenance and other factors, sublacustrine fans in the eastern steep slope zone are formed by gravity flows from different periods. The sublacustrine fan is divided into three subfacies: inner-fan, mid-fan and outer-fan. The subfacies can be subdivided into six types of microfacies: main channel, levee of main channel, distributary channel, inter-channel, levee of distributary channel and terminals. The formation of the sublacustrine fan is caused by steep slopes and triggered by earthquakes. The sedimentary model of the sublacustrine fan and its sedimentary characteristics provide a foundation for exploring and developing oil and gas in the study area.

Differential oil accumulation and its controlling factors for CFD6-4 oilfield at the steep slope belt on the southern margin of Shijiutuo Uplift in Bohai Sea

Biomarkers and stable carbon isotopes suggest that oils accumulated in CFD6-4 oilfield are mixed type by oils derived from source rocks of the third (Es3) and first (Es1) members of Shahejie formation, except the ones from the reservoirs in the third member (Ed3) of Dongying formation of Well A5, which are Es3-derived. Fluid inclusion and basin modeling analyses elaborate two charging periods in CFD6-4 oilfield. The first period initiates at about 7.5 Ma driven by hydrocarbon generating and expelling of source rocks, and another one occurred at about 5.0–2.0 Ma caused by the Neo-tectonic activity in Bohai Sea. The Paleogene fan deltas and their direct contact with source rocks define oil horizontal accumulations in CFD6-4 Oilfield. Due to the poor continuities of fan deltas, oils mainly accumulate near source, leading to oils rich in the east with poor in the west. And oil vertical accumulations in CFD6-4 oilfield depend on fault activity during Neo-tectonic activity in Bohai Sea and sandstone carrier bed of the Guantao (Ng) formation. Oil accumulations in the Paleogene and Neogene strata lies on the activity rate of fault connecting the Paleogene oil favorably accumulating reservoir and the Ng sandstone carrier, because it moved at 5 and 25 m/Ma during Neo-tectonic activity and could be both barrier to and avenue of oils. Furthermore, the oil accumulations in the Neogene strata are restrained by the structural features of the Ng sandstone carrier. Therefore, in order to improve the exploration success rate at the steep slope zones in Bohai Sea, the distribution of fan deltas in the Paleogene strata, thveir contact situations with oil source rocks, and the activity rate of connecting faults during the Neo-tectonic activity should be taken into account.

Analysis of sedimentary facies characteristics and source reservoir allocation of fault depressed Lake Basin—Taking Xu-Xi area of Songliao Basin as an example

At present, Xu-Xi area is in the primary stage of exploration, and its sedimentary characteristics and distribution law are not clear. In this study, based on the analysis of regional structural background, through core observation and description, grain size analysis, well logging and seismic and other technical means, the sedimentary facies types and spatial distribution characteristics of Shahezi Formation in Xu-Xi area are studied. It is believed that there are three types of sedimentary systems in Shahezi Formation in the study area: Fan Delta, braided river delta and lake. The fan delta is mainly developed in the steep slope belt in the west of Xu-Xi area, while the braided river delta is mainly developed in the gentle slope belt in the East and the axial belt in the north, and the lake facies is developed in the depression belt between them. On the whole, the sedimentary pattern shows the obvious characteristics of “North-South block, east-west belt”. From the West steep slope to the East gentle slope, the sedimentary facies type transits from fan delta to deep lake semi deep lake and then to Braided River Delta. Combined with the study of reservoir forming conditions, it is found that the sand bodies of fan delta front and braided river delta front have good reservoir physical properties and are often adjacent to large-scale high-quality source rocks in the upper and lower or lateral directions. The reservoir forming conditions are superior and are favorable for oil and gas accumulation facies belt.

A near-shore clastic-carbonate mixing mode in a continental rift basin (early Oligocene, eastern Shijiutuo Uplift, Bohai Bay Basin, China): sedimentology, reservoir characteristics and exploration practice

A comprehensive sedimentary and reservoir analysis was conducted based on seismic, well logging, core and relative test data, taking Members 1 and 2 of Shahejie Formation of the early Oligocene in the steep slope belt, eastern Shijiutuo Uplift (STU), Bohai Bay Basin (BBB) as a case. The study indicates that a near-shore mixed fan deposit formed in the study area and developed characteristics and pattern of a high-quality reservoir. The mixed clastic-carbonate rocks constitute Members 1 and 2 of Shahejie Formation which developed along the steep slope belt and is named as a near-shore mixed fan. The mixed fan of the study area is mainly composed of microfacies of proximal channel, mixed deposited channel, mixed clastic beach, mixed bioclastic (grain) beach, with vertical multi-stage superimposition feature, and basically a similar shape as modern near-shore fans. It constitutes a new depositional type developing in the steep slope belt of a characteristic and complex lacustrine rift basin in the study area. This mixed fan in the steep slope of eastern STU is controlled by comprehensive factors including tectonics, clastic material supply, climate, palaeogeomorphology and hydrodynamic conditions. The reservoir quality of Members 1 and 2 of Shahejie Formation of eastern STU is, however, actually controlled by the sedimentary environment and diagenesis processes. Coarse-grained mixed rocks of near-shore fans, rich in bioclastics, can form excellent reservoirs, characterized by resistance to compaction, easy to dissolution, little influenced by burial depth and high production of oil and gas, which enable them become key exploration targets of medium-deep strata of BBB. Analyses of high-quality reservoir, its controlling factors and the oil and gas exploration implications of the near-shore mixed fan developing in the study area give a deeper insight into discussions of the same type of mixed rocks of other lacustrine rift basins worldwide.

Tectono-stratigraphy characteristics of the Lower Cretaceous Saihantala Sag in the Erlian Basin, China: Normal faulting and controls on the depositional variability

Tectonism is one of the most important factors controlling the formation of sedimentary stratigraphy in lacustrine rift basins, but the controlling effects need to be further clarified. Based on 2D and 3D seismic data, well logs, logging data, and core data from the Saihantala Sag, our study aimed to analyse the fault system of the sag, determine its tectonic evolution process, establish the faulting pattern of the sag, recognise its depositional systems, and finally discuss the relationships between the tectonic factors and the basin evolution and sediment filling process.Four types of depositional systems including fan-delta, braided-delta, near-shore subaqueous fan and lacustrine deposits were recognized in the Saihantala Sag based on an integrated analysis of the well logs, cores and seismic data. The results indicated that 1) The Xilin fault is the eastern boundary fault of the Saihantala Sag, and its subsections and connections controlled the stratigraphic pattern of the sag. 2) tectonic evolution controlled the evolution of the lake basin and depositional systems in the Saihantala Sag, especially the deposition of near-shore subaqueous fan and the connection and retrogradation of braided-delta and fan-delta systems. 3) The tectonic evolution also controlled the filling pattern of the Saihantala Sag, which features two sets of braided-delta deposits in its gentle slope belt and one set of fan-delta deposits and two sets of near-shore sub-lacustrine fan deposits in its steep slope belt. 4) All three tectonic belts (Zhabu, Yihe and Saisi) mainly experienced a middle-late activated single-cycle faulting pattern, which controlled the distribution of braided-delta front deposits and lenticular sandy bodies.

Characteristics of transfer structure and hydrocarbon accumulation control of Fula western steep slope belt in the Fula sag, Muglad Basin, Sudan

Characteristics of transfer structure and hydrocarbon accumulation control of Fula western steep slope belt in the Fula sag, Muglad Basin, Sudan

Analysis of continental shale gas accumulation conditions in a rifted basin: A case study of Lower Cretaceous shale in the southern Songliao Basin, northeastern China

Abstract The analysis of shale gas accumulation conditions is the basis of shale gas exploration. Previous research has mainly focused on the Upper Cretaceous shale in the Songliao Basin of northeastern China, and few studies have analyzed the Lower Cretaceous shale in this area. Based on an integrated analysis of the sedimentary facies, shale samples and seismic interpretation from the Lower Cretaceous Shahezi, Yingcheng and Denglouku Formations of the Fulongquan Depression in the Songliao Basin, the distribution of the shale and the generation, accumulation and preservation conditions of the shale gas are evaluated and discussed. Core observations and sedimentary facies analysis show that the sedimentary facies types in the Shahezi, Yingcheng and Denglouku Formations are mainly fan delta, braided river delta, sublacustrine fan and lacustrine facies. The semideep lake, shallow lake and delta front sedimentary subfacies are favorable for the deposition of shale. The study of the shale distribution indicates that the shale was mainly deposited near the maximum flooding surface of the third-order sequences, and the shales in the Shahezi and Yingcheng Formations are darker and thicker than those in the Denglouku Formation. In addition, the shale in this study area is laterally distributed in the depression belt and the steep slope belt near the boundary faults of the basin. According to the geochemistry tests, rock properties and mineral constituents, the shales in the Shahezi and Yingcheng Formations have favorable gas generation potential. Compared with the properties of the major gas-producing shales in China and the United States, the shales in the Shahezi and the Yingcheng Formations are thick and have a favorable organic matter type, high average organic matter abundance, high organic matter thermal maturity that reached a high maturation stage, high porosity and high gas contents; these properties meet the standards of industrial exploitation, and these shales can therefore be considered favorable targets for unconventional energy exploration.

Discovery of Wanyuan-Dazhou Intracratonic Rift and its significance for gas exploration in Sichuan Basin, SW China

Abstract New understandings of the geology of Sichuan Basin were achieved in the progress of natural gas exploration in the Sinian-Cambrian strata in Sichuan Basin. An NE trending intracratonic rift was found in the Wanyuan-Dazhou area, northeastern Sichuan Basin. Based on seismic data interpretation, outcrop data and regional tectonic background analysis, we studied the boundary, distribution, formation and evolution history of Wanyuan-Dazhou rift. The following findings were obtained: (1) The seismic section indicates that a steep-slope belt indicating platform margin facies was developed during the deposition of the first and second Member (Z2dn1-Z2dn2) of the Sinian Dengying Formation. The rift generally strikes NE. (2) The thicknesses of the first and second Member of the Dengying Formation are thicker than the third and fourth Member (Z2dn3-Z2dn4) at the periphery of the rift and vice versa inside the rift. (3) The rift formed during the deposition of Z2dn1-Z2dn2 in the Sinian. Filling and subsidence occurred during the deposition of Z2dn3-Z2dn4 in the Sinian. The shrinkage of the rift happened during the deposition of the Lower Cambrian Maidiping Formation – Qiongzhusi Formation. Regional extension and uplift in the Nanhua controlled rift formation. The discovery of the Wanyuan-Dazhou rift changed the traditional understanding of the tectonic and sedimentary framework of Sichuan Basin in the Sinian and Early Cambrian. The periphery of the rift has significant potential for natural gas exploration due to its superior natural gas accumulation conditions in the Sinian.

Tectonic Characteristics and its Effects on the Control of the Oil and Gas Accumulation in Bayanhushu Depression

In this paper, it makes full use of seismic and Wells in the single well data on the basis of prede- cessors' research. On the basis of structural geology theory as the instruction, the detailed characteristics of structure and fracture were studied in this paper. Based on the fracture characteristics, the authors studied the basement depth and the development of hydrocarbon source rocks characteristics, and the restoration method of sag tectonic evolution characteristics are analyzed as well. The authors also discussed the construction control of the oil and gas distribution rules. Through this research, the authors found out the regional tectonic framework, the fault distribution and the tectonic evolution stage, illustrated the structure of oil and gas accumulation condi- tions, the accumulation mode, and the distribution rule of control law. Through prototype basin restoration, the authors given concerns about the early depression and tectonic background of stuck faults control source, the distribution of sand body, the western steep slope fan delta and gentle slope belt of eastern braided river delta front sand body is favorable reservoir. Depression after Nantun group and the end of the Yimin group at the end of the reversal of the two big changes, ring between the concave structure development, favorable traps formed early reverse changes, late reverse change on the early formation of trap destruction; reservoir by the late Nantun group is formed by the reverse change control in the construction of the steep slope belt wing structure oil and gas enrichment.

Fluid Flow of Daxing Conglomerate in Langgu Depression and its Controlling Effects on Reservoir

The Daxing conglomerate in Langgu depression is typical glutenite fan sediment in steep slope belt of faulted basin. By analyzing the data of core, petrographic thin section, scanning electron microscope and “X” ray diffraction, we’ve found that fluid flow controls the reservoir development to a certain degree. The results show that there are four types of fluid affecting the reservoir of Daxing conglomerate-meteoric fresh water, organic acid, alkaline fluid and abnormal high pressure fluid. We’ve studied fluid flows in different diagenetic stages combining the observation of clay mineral and carbonate cement and we find that two acid fluid events and abnormal high pressure fluid have determined the development of three pore belts in the longitudinal section.

Sequence Filling Patterns and Petroleum Implications of Lower Cretaceous Series in South Tamtsag Depression

Sequence stratigraphic study of Lower Cretaceous strata in the South Tamtsag Depression of Mongolia identifies one second-order and four third-order sequences. These sequences are controlled by syndepositional fault activities at the slope-break zone of the lacustrine basin. At the initial rift stage of the basin, creation of accommodation space outpaced sediment accumulation and the lake level fell below the structural slope-break zone. Sequences formed in this stage can be divided into lowstand, transgressive and highstand system tracts. At the main rift stage, lake level did not fall below the structural slope-break zone and sequences formed in this stage consist of transgressive and regressive system tracts. Vertical stacking patterns within sequences are expressed by cyclic occurrences of alluvial fan, fan delta and delta deposits in the gentle slope belt located in the western part of the basin, and fan delta, nearshore subaqueous fan and lake deposits in the steep slope belt in the eastern part of the basin. The distribution of sandstone units in the sequences, which are the major target reservoirs, can be easily recognized through the combination of sedimentology analysis and interpretation of seismic reflection profiles. The slope-break zone of the lacustrine basin, which can be identified from seismic reflection profiles, played critical roles on the thickness and lateral extension of the sandbodies in the basin, and thus is the main controlling factor of petroleum reservoirs in the basin. Because mudstone and oil shale developed during the transgressive system tracts are hydrocarbon source rocks and cap rocks, deltaic sandstone units and incision channel fills are the most important petroleum reservoirs in the Tamtsag basin. Finally, this paper divides four types of subtle traps according to the petroleum accumulation model for future exploration.

Provenance System Analysis of the Nantun Formation of the Huhehu Sag in Hailar Basin

Abstract:Huhehu Sag is a new battlefield for the future oil and gas exploration in the Hailar Basin. At present, exploration of this area stays at a low level and the understanding of the provenance system of the Nantun Formation is comparatively weak. So, to carry out the detailed depiction of the provenance system in the study area is significant for the sedimentary system reconstruction, for the prediction of reservoir distribution and favorable areas. Based on comprehensive analysis of paleogeomorphy, light‐heavy mineral and seismic reflection features, sandstone thickness and sandstone percentage, this paper analyzes in detail the provenance system of the Nantun Formation in Huhehu Sag from various angles and levels. It shows that provenances of the Huhehu Sag mainly originated from the eastern Xilinbeier Salient and western Bayan Mountain Uplift. The provenances surrounding the lake basin converged to the center of the basin, among which, the steep slope belt and the gentle slope belt are the main provenances, and the southern uplift is a secondary one. The distribution framework of the provenance fundamentally determines the distribution rules of the depositional system and its sand body developing during the Nantun period.

Syndepositional fault controlling on depositional filling of the third member of Shahejie Formation in Beitang sag

In the continental lake basin whose structures were extraordinarily active, tectonism is an important factor in controlling the sequence and the depositional filling of the basin. This article reports the assemble patterns of syndepositional fault in the third member of Shahejie (沙河街) Formation in Beitang (北塘) sag. The results show that the comb-shape fracture system and the fracture transformation zone were developed in Beitang sag. These assemble patterns obviously controlled the sand-body and spatial distribution of sedimentary system. However, the steep slope belt of fault terrace, the multistage slope belt and the low uplift gentle slope belt controlled the development of sequence styles. Analyses of the spatial-temporal relationship of the assemble pattern of syndepositional faults and the sedimentary system help predict the favorable exploration zone.

Depositional characteristics of lake-floor fan of cretaceous lower Yaojia Formation in western part of central depression region, Songliao basin

Based on the integrated subsurface data, including those of over 600 m drilled cores, more than 30 drilled wells and 600 km2 three-dimensional (3D) seismic-reflection data of the study area, the characteristics of the lake-floor fan of lower Yaojia (姚家) Formation have been clarified. An evident lacustrine slope break and a steep slope belt developed in the west of Songliao (松辽) basin during depositional period of Qingshankou (青山口)-Yaojia formations (K2). The slope gradient was about 15 m/km. During the depositional period of lower Yaojia Formation, the lake shrank and the shore line of the western Songliao basin shifted to the lacustrine slope-break. The wedge-shaped sediment body, which is interpreted as the lowstand system tract of SQy1 (LSTy1), developed in the area below the slope-break. The LSTy1 is pinched out in the west of the study area. As to the thickness of LSTy1, it is thicker in the east with 50 m in its thickness than in the west. The LSTy1, rich in sandstone, can be divided into lower part LSTy1L and upper part LSTy1u based on two onlap seismic reflection phases, and core and logging data clearly. The various sediments’ gravity flow deposits developed and the complex of lake-floor fan formed in the LSTy1 under the slope-break in the western part of the central depression region. The lake-floor fan consists of various sediments’ gravity flow deposits, including: (1) turbidity deposits with characteristics of Bouma sequences; (2) sand-bearing muddy debrite dominated by mud and mixed by sand; (3) mud-bearing sandy debrites characterized by dominated sand and mixed by mud; (4) sandy debris laminar flow deposits with massive or inclined bedding, and (5) sandy slump deposits developed as deformational sedimentary structure. During the lower lake-level period (LSTy1L), the western clinoform region was erosion or sediment pass-by area; the terrigenous clastic was directly transported to deep-water area, converted to channelized sandy debris flow, and combined with slump derived gravity flow deposited in the lower part of the slope and the deep depression region, which then formed lake-floor fans. During higher lake-level period (LSTy1u), terrigenous clastic deposited in the upper part of the slope, and formed deltaic depositional system; while in the lower part of the slope, the deep depression area was filled by lake-floor fans which were mainly slump deposits. The sandstone bodies of the lake-floor fan are the favorable targets of subtle oil-bearing traps in the western part of the central depression region.

Two Dimensional Common Reflection Surface Stack Based on the Fresnel Zone

AbstractCommon Reflection Surface (CRS) stack can improve signal‐to‐noise ratio of seismic signals. However, the key issue to improve resolution is how to set CRS stack aperture. In this paper, we discuss the characteristics of Fresnel zone during the reflection of seismic waves and suggest that the Fresnel zone under undulate layerinterface condition can be approximated with ellipse. Then we bring forward a method to determine the aperture size of CRS stack based on Fresnel zone and use it to deal with seismic section of the steep slope belt in Miyang Depression. The results show that Fresnel zone based CRS stack improves both the signal‐to‐noise ratio and resolution because the stacking aperture determined by Fresnel zone maximizes the superposition energy of seismic signals. In addition, deep weak seismic information is preserved and displayed, while normal CRS stack can only improve the signal‐to‐noise ratio and the shallow seismic information.