| The Pinghu Slope High Structral Belt(PSHSB)is located in the central of the western slope of the Xihu Sag and contains rich oil and gas resources.During the rifting period,the basin was featured by intense faulting,and sedimentary filling of the Eocene Pinghu Formation was controlled by syn-depositional fault activities.3D seismic,well logging,and core data were applied as to establish the sequence stratigraphic framework in this study.The types of sedimentary facies,the Paleogene basin-filling pattern and the syn-depositional fault controlling effects were discussed.Furthermore,the prediction of favorable hydrocarbon accumulation zones in the study area was made,which will provide theoretical support for the oil and gas exploration in the PSHSB.According to the sequence-stratigraphy theory,the three 3rd sequences were recognized in the Eocene Pinghu Formation of the rifting stage and two 3rd sequences were recognized in the Oligocene Huagang Formation of the thermal-subsidence stage.Through the analysis of thickness maps of different periods,it is found that there are two large NE-direction subsidence centers in the northeast and southeast of the slope high belt during the deposition period of Pinghu Formation,and the distribution direction is NE-SW direction.During the depositional period of the Huagang Formation,affected by the basin tectonic inversion and denudation,the deposition and subsidence centers were shrined,and the direction was changed to the E-W direction.Through the identification of seismic facies,logging facies and the analysis of core facies,six types of sedimentary facies were identified in the study area,including: fan delta,braided-river delta,tidal flat,lacustrine and sublacustrine fan.The Pinghu Formation is mainly composed of braided river delta facies and tidal flat facies.During the SQ1 period,the study area was mainly composed of braided river delta deposits,and the tidal flat deposits were relatively limited.During the SQ2 period,the fault control on sedimentary filling was weakened,and the depositional environment was mainly delta front and tidal flat facies.During the SQ3 period,the depositional environment was stable,and a large number of distributary channels,interdistributary bays,and sand bar sedimentary microfacies were developed.The seismic profile styles of the fault system in the study area mainly include: compound "Y" shape faults,flower structures,comb shapes,fault steps and feather shapes.Under the combined effects of paleotopography and paleo-geomorphology and syn-depositional fault activities,three types of structural slope break belts have formed,including: the same-dip fault-step slope-break belt,the opposite-dip fault-step slope-break belt,and the reverse-dip fault step slope-break belt.These structure slope belt controled the evolution and development and sedimentary filling of the Pinghu slope in the study area.Four favorable traps were predicted,including:(1)The high-belt structure-lithologic pinch-out composite trap in the slope west of Well XH-12-1;(2)The fault block trap in the high part of the paleo-uplift east of Well XH-12-1;(3)The local structure of XH-23-1 reverse fault step Traps;(4)The lithologic-stratigraphic traps in the partial depressions of the high zone on the west side of the slope of Well XH-16-1 and the upper ultra-unconformity pinchout zone. |
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