Research On High Resolution Sequence Stratigraphy And Microfacies Of Yan'an Formation In The Jiyuan Oilfield

The Ordos basin is a steady subsiding craton suprimosed basin with complex depression, being rich in oil and gas resources. The Jiyuan oilfield is in the east of the Ordos basin with very particular geologic condition of oil and gas. The palaeogeomorphologic reservoir of Yan'an formation is small and scarce. By far, compared with other areas in Ordos basin, few successful exploration have taken place in Yan'an formation, Jiyuan oilfield. It need more attention and deeply study to make a breakthrough.High resolution sequence stratigraphy method which is also applicable to continental strata and sedimentology theory is applied in order to divide different hierarchical base level cycles, that are used to establish sequence stratigraphic framework, discuss the spatial distribution of sand body and the sedimentary microfacies and at last, confirm the evolutionary process as well as the sedimentary model. The research can provide theory to the study on reservoir, and offer scientific basis for the develop deployment in future.Based on the lithologic and electric data of 300 wells in Jiyuan oilfield , such as core, well log and drilling, combined with the change of palaeostructure, palaeoclimate,palaeogeomorphology and supply of sediment, hierarchical base level cycles are recognized in reference to one super long-term base level cycle(SLSC1), 3 long-term base level cycles (LSC1-LSC3) and 10 middle-term base level cycles (MSC1-MSC10) according to base level cycle theory. The first long-term base level cycle, which developed from Yan10 substage to Yan6 substage, is made up of 6 middle-term base level cycles, the second long-term base level cycle, corresponding to Yan5 substage and Yan4 substage, is composed by 2 middle-term base level cycles, and the third long-term base level cycle, built up from Yan3 substage to Yan1 substage, also can be further divided into 2 middle-term base level cycles. The short-term base level cycles, which construct the middle-term base level cycle, can be further classified into 2 types, that are upward deepening non-symmetry cycle sequence, taking place in braided river and braided delta plain and upward thinning non-symmetry sequence, which occur in braided delta front and predelta. Finally, the stratigraphic framework of Yan'an formation is established, which shows the strata of Yan'an formation is denuded.The sedimentary systems and the sedimentary facies of Yan'an formation in Jiyuan oilfield is complex. The sedimentary system in Yan10 substage is large braided river with deep channel bar, form Yan9 substage to Yan5 substage, the sedimentary system is multiple fluvial-shallow lake-braided delta. In Yan4 substage, the sedimentary system is anastomosing delta. Fluvial system developed once more in Yan3 substage to Yan1 substage. The main subfacies in the research area is braided delta plain and the dominant microfacies is braided channel, channel bar, distributary channel as well as the interchannel and the overbank deposit in braided delta plain.The characteristic of the shallow lake is emerging quickly but dying slowly. Accordingly, the evolution of sediment in the area can be divided into 5 substage, that is fill and level up substage, fluvial-lake substage, shallow lake obliteration substage, shallow lake discussed substage as well as uplift and denuded substage.