Sedimentary System And Formation Mechanism Of Deep-water Channel Complex

Since the twenty-first Century, with the application of high-precision3D seismic technology, the deepwater channel depositions have gradually become the important area of oil and gas exploration in deep water. At present, the study on sedimentation process of deepwater channel is still poor, and the division and sedimentary model of the deepwater channel depositional system remain a problem of much controversy. Research on the deposition of deepwater channel started late in China, and the research means mainly based on the3D seismic data, are quite unitary. However, the high-resolution3D seismic can only reflect the large scale characteristics of the deepwater channel, not the finer scale of the internal lithology and filling structure. Therefore, this paper selects two field profiles of the Middle Ordovician Lashizhong formation at Zhuozhishan area, Inner Mongolia and the Middle Triassic series in Tianlin area, Guangxi, combined with the seismic profile of the Miocene deepwater sediments at the lower Congo basin in West Africa, this paper discusses the deepwater channel depositional system, summarizes the types and characteristics of sedimentary units and Internal filling structures of deepwater channel, and establishes sedimentary model.The main research contents as follows:①Study on the sedimentary system of the deepwater channel, with two field geological profile and a seismic section as the research object, this paper discusses the deepwater channel morphology, types and characteristics of sedimentary units;②Study on the filling structure of the deepwater channel, including the study of the transformational mode of sediment gravity flows, the deepwater channel filling processes and filling structure evolution etc;③Study on the formation mechanism of the deepwater channel depositional system, combined with the geological section and seismic section of the deepwater channel, analysis the main control factors, discusses its formation mechanism.This paper provides a series achievements as follows:(1) Based on field geological sections and seismic profiles, this paper summarizes the deepwater channel depositional system, considers the deepwater channel sedimentary system mainly includes five sedimentary units, namely the channel, levee, lobe, crevasse splay and abandoned channel. From the shelf edge to the abyssal plain, the deepwater channel on upper slope break zone is connected with the canyon, with obvious erosion and low sinuosity. With the decreasing of the slope gradient and erosion abillity, the high sinuosity channel-levee system developes, the lobes develop at the end of the channels. Besides, crevasse splay and abandoned channel deposits mainly occur in the high sinuosity channel belt.(2) Combined with the geological section and seismic section, this paper analyzes the conversion of gravity flows from slide, slump to debries flow then to turbidity current, and introduces the transformational mode of sediment gravity flows. Sediment gravity flows from the beginning to the end deposited, have the general experience of four fluid conversion processes, namely the slide, slump, debris flow and turbidity current. Each pattern is not completely separated, but a relationship gradually transition.(3) The filling process of deepwater channel are divided into three stages:initial formation stage, erosion-filling stage and abandonment stage. What is more, the vertical filling model of deepwater channel proposed by Mayall et al has been modified, and consider the low sand-mud ratio section at the top should belong to the overflow deposit. Conbined with the transport process of sedimentary gravity flow, a model of filling structure evolution of deepwater channel is established.(4) The main controlling factors of deepwater channel depositional systerm are analyzed, and then its formation mechanism is summarized. The deepwater channel depositional system is mainly affected by climate, sea level change, tectonic activity, source recharge, and occurs in a certain trigger mechanism.(5) Establish the sedimentary model of deepwater channel as follows. In plane, mass transport deposit can form channel deposit when encountering the canyon-channel system, and the scale of channel is controlled by the sediment supply. In section,①the shelf edge and upper slope are mainly the slide and slump deposits;②the proximal channel are mainly debris flow deposits, with a few turbidity and levee deposits;③the distal channel are mainly the turbidite and levee deposits;④lobes can be divided into the proximal ones and the distal ones. The proximal lobes are mainly composed of distributary channel deposition and lobe deposition, while the distal lobes are mainly composed of thin interbedded of sand and mud lobe deposition.