The Reservoir Characteristics Of The Cretaceous Carbonate Rocks And The Differential Enrichment Of Hydrocarbons In The Ahdeb Oilfield,Central Iraq

Ahdeb oilfield in central Iraq is the overseas oil field taken over by China from2008.The main oil producing zones are distributed in the Upper Cretaceous Khasib,Mishrif and Rumaila Formations and the Lower Cretaceous Mauddud Formations.The porous limestones containing a lot of skeletons and fossils are developed in the Cretaceous reservoirs.There are three structural highs developed in the oil field,among which the top oil layer Kh2 contains oil at the three highs,but the zones below layer Kh2 contain oil only in the east,and the two highs in the west are basically water layers.There is a difference in the degree of petroleum enrichment in the west and east.Due to the poorly understanding of the formation and evolution of Cretaceous limestone reservoirs and the lack of research on hydrocarbon enrichment characteristic in the Ahdeb oilfield,the development of oil and the method of enhanced production of oil is restricted.On the basis of core analysis,thin section observation,organic geochemical analysis of crude oil and seismic interpretation,this study aims to clarify the characteristics of Cretaceous reservoir and the controlling factors of reservoir development in this oilfield,and further analyze the factors and process of hydrocarbon accumulation,so as to reveal the hydrocarbon enrichment patterns in this oilfield.During the late Early Cretaceous to the early Late Cretaceous,the sedimentary facies in the Ahdeb oilfield are carbonate platforms.On the regional unconformity surface,with the rise of sea level,the sedimentary facies evolved from carbonate ramp to carbonate platforms.The ramp facies in the study area mainly develops inner ramp subfacies,with marl and mudstone.Carbonate platform includes restricted platform,plate and open platform subfacies.There are various types of shoals in restricted and open platform,dominated by intraclastic shoal and bioclastic shoal,and which have the characteristics of multi-phase superposition vertically.The lithofacies from the Khasib to Mauddud formations are mainly micritic granular limestone,microsparry granular limestone and granular micritic limestone,and the main void spaces are intergranular dissolved pores,algal mold pores and coelomopores.The formation of reservoir void spaces was mainly related to sedimentation,syngenetic neomorphism and fabric selectivity dissolution.The porosity of each layer ranges from 11.7%to 26.7%,and the permeability varies from 0.007 to 14.639×10^-3μm².The pore throat radius has the characteristics of bimodal distribution.The pore throat coordination number of granular limestone is higher than that of micritic limestone,so it has higher permeability.The porosity,permeability and pore structure of the lithofacies in the algal shoal are the best.The pore structure of the intraclastic shoal is secondary,but the porosity varies widely.The best reservoir qualities are in Kh2 and Ru3 layers,with Ⅰ-Ⅱ type reservoirs,while Mi4,Ru1 and Ma1 layers have Ⅰ-Ⅲ type reservoirs,and Ru2a and Ru2b layers have Ⅱ-Ⅲ type reservoirs.Through the reservoir comparison horizontally,the distributions of reservoirs except Ru2a layer are relatively stable.Based on the analysis of oil source,structural morphology and its evolution,and the study of fault development characteristics,the geological characterisitcs of hydrocarbon accumulation in Ahdeb oilfield are revealed.The saturated hydrocarbon chromatography of the oils from seven layers is pre-peak type,and the organic matter type is mainly Ⅰ-Ⅱ type,with C₂₇ cholestane>C₂₉ cholestane>C₂₈ cholestane;The22S/（22S+22R）ratio of C₃₁ and C₃₂ homohopance is about 0.56,indicating that they are mature marine organic matter.The source of oils from the seven layers in Ahdeb oilfield is basically the same,which is related to the Chia Gara Formation of Upper Jurassic and Lower Cretaceous,with the characteristics of hydrocarbon migration from the lower source rocks.There is a small amount of terrigenous material input near the fault in the central part of the study area,which is contributed by the hydrocarbon sources of the Ratawi and Zubair Formations of the Lower Cretaceous.The tectonic evolution of the study area happened in two important periods:the eastern anticline formed by the closure of the eastern Neo-Tethys Ocean before Late Cretaceous,and the western anticline formed under the influence of the Zagros orogeny after Miocene.There are two groups of faults developed in the study area,namely,the NWW-trending normal fault with the unpenetrated Nahr Umr Formation and the NE-SW strike-slip fault.Under the influence of the NE-SW strike-slip system in the middle of Iraq,the strike-slip fault developed in the middle of Ahdeb oilfield,which can connect the hydrocarbon source.Strike-slip faults developed in the Cenomanian and reactivated in the Late Cretaceous and Miocene,respectively.The hydrocarbon accumulation process in Ahdeb oilfield is revealed through the analysis of hydrocarbon charging history and the hydrocarbon migration direction.Based on the measurement of homogenization temperature and salinity of brine inclusions,Fourier transform infrared spectrum analysis of oil inclusions,and measurement of the carbon and oxygen isotopic of calcite cements,four times of hydrocarbon charging occurred in the study area,which occurred in the Cenomanian95-97Ma,the Campa to Meslichtian 71-79Ma,the middle Miocene 14 to 15Ma and the late Miocene10Ma.Among them,the main accumulation period was 71-79Ma in the Late Cretaceous.According to the analysis of crude oil properties,carbazole biomarker compounds and fluid potential,it is considered that the crude oil from the Khasib Formation migrated from bottom to top and from east to west,and the time of migration from east to west was roughly in the late Miocene.According to the above studies,the effect of hydrocarbon accumulation on the differential enrichment of hydrocarbon in the Cretaceous reservoirs in Ahdeb Oilfield is discussed.The distribution of reservoirs influenced the lateral distribution of oil during the early oil charging period.The differential charging of petroleum horizontally was controlled by the assembly of source,reservoir and trap.The differential enrichment of petroleum vertically was controlled by the passway system and the assembly of source and reservoir.The differential physical property of oil was influenced by the migration from east to west.Finally,the models of differential enrichment of petroleum in the Ahdeb oilfield were established in the four times of hydrocarbon accumulation,e.g.（1）Initial charging period:The heavy oil in Ru3 layer and Ma1 layer was charged along the active fault in the eastern high in the Cenomanian period;（2）The stage of oil enrichment in the eastern area:The large-scale hydrocarbon generation in the Meshrichter stage resulted in the seven zones from Kh2 to Ma1 layer bearing oils splited by some interlayers;（3）The stage of the Kh2 zone in the east structure filled with oil:In the middle Miocene,during the early Zagros movement,high mature oil charged into the Kh2 zone along the fault;（4）The period of oil in the Kh2 zone adjusting from east to the west:The oil in the Kh2zone migrated from east to the west in late Miocene.