Exploration Theory And Practice Of Lithologic And Stratigraphic Reservoirs In Erlian Basin

Erlian Basin is a group of continental fault-depression lake basins of LowerCretaceous epoch developed on the basement of Hercynian fold belt in InnerMongolia-Great Xingan Mountains. The basin is composed of 53 sags and 21horsebacks. The main sedimentary formation in the basin—Lower CretaceousBayanhua group—is divided into three Formations of Saihatala, Tengger and Aershanfrom top to bottom. The sags are separated distinctly from each other with poorcommunication, and independent sedimentary system for each sag developed with thesedimentary characteristics of multi-source, proximal source, small water system,coarse fragment, quick facies change and narrow facies belt. The dark mudstonesdeposited during two massive lacustrine transgressions of Teng-1 Member andAershan Formation are the two sets of major hydrocarbon source layers in the basin.Each sag is a relatively independent petroleum system for hydrocarbon generating,migrating and accumulating. Its unique structural and sedimentary features andreservoir forming conditions are favorable for the forming of lithologic andstratigraphic reservoirs.Several innovative understandings have been obtained in the course ofexploration study and practice of lithologic and stratigraphic reservoirs in ErlianBasin. The first is that oil and gas distribution has a feature of "complementarity" inhydrocarbon-rich sag in petroliferous basin, which dues to the coupling of variouselements such as geological setting, reservoir forming condition, and reservoirforming mechanism of various types of oil and gas pools occurred in the basin. Thesecond is that lithologic and stratigraphic reservoirs are mainly distributed along "lakeflooded surface, unconformable surface and fault surface", namely "three surfacescontrol reservoir". The third is that lithologic and stratigraphic reservoirs maydistribute in both high potential area and low potential area, i.e. can develop not onlyin high system tract but also in low system tract. The fourth is that lithologic reservoiris characterized by facies control, stratigraphic reservoir is characterized by layercontrol. And the fifth is that only pre-existing and syngenetic slope-breaks have theeffect to control facies, sand and reservoir. The presentation of these innovativeunderstandings has broken traditional recognitions such as "annular depressiongathering oil", positive structural belt gathering oil, low potential area controlling oiland low system tract enriched with oil.To explore lithologic and stratigraphic reservoir in hydrocarbon enrichment sagwith high degree of exploration, the exploration concepts must be changed, and the new exploration concepts must be established, namely various types of reservoirs inhydrocarbon enrichment sags are "distributed in various structural elements, variousplays, various series of strata, various sedimentary systems, various sedimentaryfacies belts and various types of reservoirs in a mode of three dimensional duplexoverlaying and connecting with each other". Exploration thoughts, therefore, havebeen extended from structural reservoir to lithologic and stratigraphic reservoir; frompositive structural belt to negative structural area; from high part of structural belt tothe flank; from structural belt to lithofacies belt, slop-break belt, overlap belt anderosion belt; from annular depression to trough; from mono reservoir type to multireservoir type. The exploration procedure has been changed from "determine trough—select belt—preliminary prospecting—evaluation" previously worked out aimed atstructural reservoir exploration to nowadays "determine trough—determineexploration domains—select belt—preliminary prospecting—evaluation". Duringexploration practice of lithologic and stratigraphic reservoir In Erlian basin, three keysectors have been emphasized, namely evaluation and optimization of favorable playsfor lithologic and stratigraphic reservoir, recognition, evaluation and optimization oflithologic and stratigraphic trap, preliminary prospecting and evaluation of lithologicand stratigraphic reservoir trap. As a result, a set of corresponding explorationtechnology and method for lithostratigraphic reservoir has been formed gradually.Innovation in exploration concept and progress in exploration technology haveresulted in great breakthrough in the exploration practice of lithologic andstratigraphic reservoir in Erlian Basin. Since 2001, scale uncompartmentalizedreserves predominated by lithostratigraphic reservoir have been discovered inBayindulan, Wuliyasitai and Jiergalangtu sag successively. The scale of third classreserves has reached over 10~8 ton, and good socioeconomic performance has beenachieved.