Tectono-sedimentary Analysis Of Nanpu Sag In The Bohaiwan Basin

Growth Structures plays an important role on control ancient relief, sedimentary processes and sedimentary facies distribution in rift basin. With the widespread ues of 3D seismic data, recently subtle tectono-sedimentary analysis of sedimentary basins has attracted much attention. Based on the expatiation of the tectonic development and sediment filling history, the sequence stratigraphic framework of the Nanpu sag, this thesis discusses the growth structures pattern, activity, evolution, forming mechanisms of three structural belts (Laoyemiao-LYM, Gaoliu and Getuo) and their controls on sedimentary processes highlighting on sediments entry, transportation pathway, sedimentary facies distribution, and then establish the model of relationship between tectonics activity and sedimentation. Together with the reservoir-cap combination, finally, this paper accesses the favorable traps distribution and presents integrated model of tectonic-sedimentation-hydrocarbon accumulations. The results could not only enrich the theory of tectono-sedimentary analysis of rift basin in eastern China but also be helpful to guide the oil and gas exploration of three belts in the Nanpu sag. The main results in the thesis are as follows:1. The LYM anticline axis trending NW-SE and plunging to the south is perpendicular to the Xi'nanzhuang fault (XNZF), so it is a transverse anticline, which is superimposed by NE trending strike slip faults. On the seismic sections, the XNZF surface curved and deformed the overlaying strata from Eocene to Oligocene, thus the LYM transverse fold is ascribed to non-planar XNZF surface origin, including the anticline formed at the fault surface salient and adjacent synclines at the concave. The strata of ES2 member-Ed formation show significant thickening from the hinge sidewards the synclines, which prove the anticline to be synsedimentary. The Nanpu sag was controlled both by NE Tan-Lu fault and by NW Zhangjiakou-Penglai strike slip faults in the geodynamic situation, since Miocene it has suffered deformation and formed a large number of NE and NW oriented faults. The strike slip faults of LYM region is part of Beipu-LYM faults system, created in the Nm-Q period.2. Gravel content is largely used to localize the ancient river channels. Such statistics of Ed1 and Ed2 members in the LYM region show a high degree of similarity; all maximum lie on the joint of the transverse anticline axis and XNZF, and the high values zone reflecting the ancient river channel basically coincided with the anticline hinge along which the value reduce slower than towards limbs. The fold hinge is thought to provide major sediment transportation pathway for long distances. The drainage in transverse anticlines may enter the basin from highs on the crosscutting of the anticline and border fault, and flow generally along anticline highs. The upper and lower slope breaks of LYM transverse fold are identified on the seismic data, the former lies on the turn of the anticline crest, the later near the limbs inflection. Combined with the analysis of seismic facies, logging, drilling, the upper and lower slope breaks are consistent with the delimits of fan-delta plain and front, fan-delta front and pro-fan delta respectively. Fan-delta plain accumulated on the anticline crest, fan-delta front on the anticline front and the limbs upper part, slumps on syncline and the Linque subsag. It can be proved that the slope breaks constrains sedimentary facies distribution. The sediment transportation with long distance on the anticline hinge result in fan-delta displaying tongue-like geometry with long axis consistent with the anticline hinge.3. Combined with reservoir properties, suitable reservoirs within the delta subaqueous distributary channel and mouth bar are predicted in the fan-delta deposited on the anticline crest. The transverse anticline superimposed by NE trending strike slip faults accounts for the principal trap-structural traps, such as anticlinal, faulted nose, faulted blocks. The LYM tectonic-sedimentation-hydrocarbon accumulation model is presented as followings:the transverse anticline exerted a major influence both on the sedimentary facies distribution of fan-delta and on suitable reservoir distribution; the strike slip faults penetrating into the deep source rocks by connecting with shallow reservoirs provide the major pathway of hydrocarbon migration; the LYM faulted anticline eventually formed abundant traps.4. We described the structural style of Gaoliu region, and concluded a "Seesaw activity" type as well as the evolution features of the boundary faults. Gaoliu region was restricted by the boundary faults of both XNZ and BGZ, as well as the GaoLiu fault, and it recorded the information of structure deformation during Es, which is of great importance to investigate the characteristics of boundary faults and the forming mechanism of Nanpu sag. It was shown from the gravity contour and horizontal slices that there were no cutting relationships between XNZF and BGZF, thus we considered it as a Xi-Bo fault belt. In this region, second-order faults were not developed, but the "cabbage" type structure and the LiuZan reverse drag anticline were identified in the southern part. The characteristics of fault occurrence, the fault activity statistics, the profile interpretation, and the stratigraphic thickness of statistics have showed striking differences between the Xi segment and the Bo segment on the Xi-Bo Fault. The BGZF was NW trending fault with a steep inclination, and it strongly acted during Es1-Ed3, with its controlling deposition center located in east Shichang subsag; the XNZF was a NE fault and displayed a Shovel-shaped to plate-like morphology, and it strongly acted during Es3-Es2, with its controlling deposition center located in west Shichang subsag. According to those features discussed above, we suggested a "Seesaw activity " type for the Xi-Bo Fault:the XNZF strongly acted during ES3-ES2, while the BGZF presented weak "activity, and especially during the sedimentary period of Es31 submember-Es2 member, the XNZF acted so strongly that the hanging wall of BGZF was tilt-lifted and suffered erosion; on top of the Es1 interface, the BGZF, acted strongly, which led the hanging wall of XNZF to be tilt-lifted. In summary, the southern part in Gaoliu region was considered to be a slope belt, while the northern part experienced an alternative variation between a deposition center and a erosion region after tilt-lifting.5. We improved the provenance system of Gaoliu area, classified steep-and ramp-type sediment stacking patterns, and established the sedimentary response model with respect to "seesaw" type of Xi-Bo fault. According to the logging and seismic reflection structure, we identify southwestern source for the new provenance near the well Gll. This source influenced the deposition of southwestern gentle slope area beneath the Es1 interface. The pattern of sediment stacking could be divided into gentle-slope type and steep-slope type. The former is mainly distributed in the gentle-slope zone controlled by tilted fault structure, the drainage covers a long distance and wide range and so is the sedimentary facies; the latter distributed mainly in the boundary of steep fault zone, the sediment accumulated rapidly and the distribution of sedimentary facies is narrow. Combination of the sediment stacking patterns, the "seesaw" activities are classified into four stages and the sedimentary response models of each stage are also established:1) Early double-break stage. The throw side of Xi-Bo fault appears as steep-slope zone short-distance deposition; 2) Middle steep-slope Xi segment and gentle-slope Bo segment stage. The sediments were transported a long distance and distributed in large area in gentle-slope Bo segment, and the sediments were quickly short-distance deposited in steep-slope Xi segment.3) Late gentle-slope Xi segment and steep-slope Bo segment. The sediments were transported through a long distance into the deep lake in Xi segment and the sediments were significantly deposited vertically.4) End double-gentle stage. Two-way sediments were supplied sufficiently, filling the most area of the lake.6. We produced hydrocarbon accumulation models associated with "seesaw" activity of Xi-Bo Fault. By changing the earlier occurrence of stratum, the later activity of "seesaw" reconstructed the earlier traps. Beneath the Es1 interface, stratigraphic unconformity trap, channel sand lithologic trap and updip pinchout trap appears in the BGZF hanging wall, and fault-lithologic trap and channel sand lithologic trap appears in the XNZF hanging wall. During Es1-Ed formation, the variation of Gaoliu region texture leads to the formation of BGZF steep slope and gentle slope and reconstruct the earlier strata and reservoir occurrence up to form new traps. For example, the unconformity traps formed in Es1 partly convert to the updip pinchout traps. Additionally, many new traps were generated in this period. The southern part of Gaoliu district develops typical traps concerning with gentle slope, such as channel sand lithologic trap, updip pinchout trap, stratum overlap traps and stratigraphic unconformity trap.7. We identify the main synsedimentary faults (the Gebei fault, Getuo broom-shaped fault system and BGZF) which restrict buried hill drape anticline. The Gebei faults developed from Cenozoic and controlled ES3-ES2 member; the major fault of the Getuo fault system formed at Eocene and controlled the formation of Caofeidian subsag. The broom structure took shape at middle of the Oligocene. It shows the arc-shaped converging towards SE and diverging towards NW direction, as a product of sinistral transtensional stress field. Getuo district always presented "two sags one arch ", however, it was controlled by different faults. The Gebei fault together with BGZF controlled the Liunan subsag, and Getuo fault controlled the Caofeidian subsag beneath the Esl interface; above the Esl, Gaoliu fault became active and controlled the Liunan subsag combined with Bogezhuang fault, meanwhile, Gebei fault acted weakly and Getuo broom-shaped fault system controlled the Caofeidian subsag.8. The relationships between faults activity and sedimentary alimentation rate has been established. Model of sandbodies transportation with drainage oblique crossing the broom-shaped faults system is proposed. The author classify the following 6 categories of steep-slope type sediments stacking patterns:up-progradation type, down-progradation type, near-progradation type, away-progradation type, aggradation type and onlap type, first four of them are with uplift-block provenance, whereas the rest lack of provenance. What's more, qualitative analysis of BGZF activity has been given. The drainage coming from estern and northern Getuo buried hill conduct behaviors of 1) cutting cross the broom-shaped fault and feeding the lower terrace; 2) transporting along the fault terrace downwards into the subsag. These lead to the broom-shaped fan delta front sands dispersing pattern and the model of sandbodies transportation with drainage oblique crossing the broom-shaped faults system is proposed. Finally, we predict that the traps related to the broom-shaped faults system, such as fault trap, faulted block trap and fault-lithologic trap are the favorable traps.9. The mechanism by which structure exerts control on sedimentation-accommodation variation has been systematically discussed. Significant influences exerted by the tectonic motion on deposition of Es1 interface have been pointed out. Beneath the Es1 interface, the basin conducted NW-SE extensional and formed a series of NE trending faults and deposition centers. On the interface, the basin conducted NW transtensional and formd the NW trending eastern Shichang subsag and Caofeidian subsag. Caused by XNZF continued extension along shearing direction of the BGZF, Nanpu sag is firstly attributed to transtensional fault-termination basin. The laws of tectonics controls on sedimentation in different evolution stage of Nanpu sag have been concluded in terms of structural slope break, fault activity intensity,structural styles and paleogeomorphology...