关键词: 远端重力流, 水平井, 深水油气, 延长组7油层组, 鄂尔多斯盆地

Abstract: The distal gravity flow sedimentary body of Chang 7₃ sublayer in the Ordos Basin is an important target for deep-water oil and gas exploration, but there is still an unclear understanding of its sedimentary scale and spatial distribution law, thus resulting in low drill ratio of sand bodies and high exploration risks in horizontal wells. Using the core, drilling, mud logging and logging data of Chengye 1 pilot well, Chengye 1 and Chengye 2 horizontal well, this paper studies the sedimentary types, distribution characteristics, sand body structure, deposition process and mode of the distal gravity flow of Chang 7₃ sublayer in the Chengye horizontal well region. Four types of sedimentation are developed in the deep-water sedimentary system of Chang 7₃ sublayer:sliding-slumping, arenaceous debris flow, turbidity flow and deep lacustrine mud deposits. The sand bodies are dominated by intermittently distributed arenaceous debris flow tongue-shaped bodies, with the thickness of 0.6-2.5 m for a single stage. The sand bodies have a stable thickness and good continuity along the provenance direction (NE), with a length of up to 800 m; in the direction perpendicular to the provenance (NW), the continuation length is mostly less than 150 m, and pinch out occurs suddenly. The gravity flow deposits in the horizontal well region of Chengye were formed by the instability, detachment and transportation of sediments in the delta front of the southwestern margin of the lake basin. The transportation of the nappe mainly experienced four stages of sliding, slumping, arenaceous debris flow and turbidity flow. The separation body of arenaceous debris flow is transported over a long distance and forms distal gravity flow sand bodies after wholly coagulating in the center of the lake basin. Sliding-slumping and turbidity flow deposits are the products of the interaction between the separation body of arenaceous debris flow and the lakebed and lake water. The structure of gravity flow sand bodies at the distal lake basin is controlled by the occurrence frequency and the single-stage scale of gravity flow events in the delta front. Isolated sand bodies are formed in gravity flow events with low occurrence frequency and large single-stage scale. Continuous superimposed sand bodies are formed in gravity flow events with high occurrence frequency but relatively small single-stage scale. The gravity flow sand bodies superimposed at intervals are the transitional deposits of the former two. Arenaceous debris flow sand bodies and deep lacustrine black shale at the distal lake basin are favorable targets for deep-water oil and gas exploration in Chang 7₃ sublayer of the Chengye horizontal well region. The development zone of fine sandstone in the Chang 7₃ sublayer in wells B522, C81, and C121 distributed in NE direction is a sweet spot for exploration, and the lower sand group of the Chang 7₃ sublayer is a section of the sweet spot. The direction along the provenance is a favorable horizontal drilling direction.

Key words: distal gravity flow, horizontal well, deepwater oil and gas, Chang 7 reservoir, Ordos Basin