Evolution of Mouth Bar Complex's Architecture in a Long-term Base-Level Cycle: A Case from the Lower Es₂ Member in Fault-Block Tuo 7, Shengtuo Oilfield

Key words: 
• Shengtuo oilfield /
• base-level cycle /
• architecture /
• mouth bar

Abstract: High-resolution sequence stratigraphic theories and the architecture analysis method are synthetically applied to study controls of long-term base-level cycles on architecture of mouth bars by taking the lower Es₂ Member in Fault-Block Tuo 7, Shengtuo oilfield as a case study. The results show that the bottom of Es₂ Member of Tuo 7 Fault-Block in Shengtuo oilfield constitutes a long-term base-level decreasing semi-cycle which can be subdivided into 8 middle-term cycles, some short-term cycles and various super short-term cycles. During long-term base-level fall, continuity of mouth bars become better, scale of distributary channels on the bars become larger and extending distance of these channels become longer; At the early stage of long-term base-level fall, combination patterns between different mouth bars in the same period are mainly by means of one mouth bar's fringe staggers on the another or mouth bars are separated by inter bar mudstone, At the late stage of long-term base-level fall, combinations between different mouth bars in the same period are mainly by means of one bar's main part overlaps another bar's fringe or one bar's fringe overlaps another's; Meanwhile, during long-term base-level fall, mouth bars' vertical connectivity become better and dip angles become smaller. Long-term base-level cycle controls mouth bars' architecture mainly through regular change of accommodation and supply rate of sediments. This study has a potentially important significance in guiding prediction of sandstones and remaining oil's distribution in this type of oil and gas fields.