The Paleotopography Restoration and Structural Modeling Based on Base Level Equilibrium Method:A Case on Abu Gabra Formation in Block 1/2/4 of Sudan,Muglad Basin

摘要

Finding oil and gas in lithologic traps is still a big challenge in rifted basin,with multi-periods tectonic subsidence,complicated fault systems and provenance supply variations,and paleotopography restoration is the necessary and essential step to study sediments characteristics and reservoir sands distribution,especially with thin sand and shale interlayers.From Neocomian to Barremian period,4500 m thick Abu Gabra formation of Muglad basin was formed in just 17 Ma,a relatively short period and thick strata thickness compared with other formations.Based on accommodation space and sediments supply(A/S)view,paleotopography restoration in each stage of faulting Abu Gabra formation was done with seismic-well tie data.Deposition happens when base level is higher than ground surface,erosion happens when base level is lower than ground surface,and deposits bypass when base level is near ground surface.Moreover,the method has advantages on free of samples limitation and basinal scale large area application.According to 2D seismic data of 3019 lines,3D seismic data of 3580 km"and 92 wells in total 48,788 km~2,structure models of five stages in AG formation of block 1/2/4 are established.(1)The average paleotopography restoration thickness from the lower AG5 to the upper AG1 stages changes smaller and smaller,from more than 600 ms to near 340 ms time-interval thick,indicating structure evolution change from rifted basin to depression basin;(2)The paleotopography restoration results in each stage of AG formation shows early erosion is mainly in the southwest and south,and later erosion is mainly in the east and the northeast,and four provenance supplies come from northeast,west,east and southwest directions;(3)Compared tectonic structure and paleotopography restoration result in each stages of AG formation,deposition center and subsidence center are not the same,especially in late uplifting period,i.e.,AG1,AG2 and AG3 stages.The dynamic balance between tectonic subsidence and sediments supply decides sands/shales distribution and good reservoir/cap rock composition.