Abstract:

Shallow water delta is a hot topic in sedimentology and hydrocarbon exploration with much attention given to depression lacustrine basins. Some scholars have studied the sedimentary evolution of shallow water delta in rifted lacustrine basins and proved that it can also develop in the basins’ recess or early rift stage; however, as a whole, the sedimentology of shallow water delta in rifted lacustrine basins was poorly introduced. Here, based on the core, well logging, seismic and laboratory analysis data, the favorable conditions of sedimentation, sedimentary characteristics and vertical evolution of the shallow water delta in Aradeiba Formation are studied in this paper, and the sedimentary evolution model of the shallow water delta in the early stage of rifting period is proposed in combination with the changes of lake level and the sediment supply. Relatively simple structure with a wide gentle terrain inherited from little basement fluctuation, warm humid paleoclimate conducive to source area weathering and sufficient source materials, and frequently fluctuating shallow paleolake level (inferred by core mudstone data) are the favorable structural and sedimentary conditions for shallow water delta development during the early 2^nd rifting period in the Cretaceous Aradeiba Formation. Comprehensive analysis results of core depositional cycles, sedimentary structures and components, grain size features and mud logging characteristics show that shallow water delta developed in the study area. As it is shown, the core is segmented by several small scour surfaces or sedimentary sections in an upward rhythm. The lower part of each segment contains medium-coarse sandstone with strong hydrodynamic sedimentary structures, such as trough and wedge cross-beddings, while the upper part deposits mostly variegated mudstone. The content of quartz, well sorted, is very high, and the grain size probability curve of sandstone mainly has the two-segment shape, showing high saltation and a certain amount of suspended population consistent with the sedimentary characteristics of shallow water traction flow. The development of distributary channels is a significant feature of the shallow delta front, featuring a variety of logging facies types, such as high amplitude box, high amplitude bell and medium/high amplitude thin box type. Another important type of reservoir sandbody, mouth-bar, shows medium/high amplitude funnel type on the logging curve. The Aradeiba Formation in the study area is divided vertically into 5 sublayers based on drilling and logging data of several oil fields in the Unity Sag. Each sublayer exhibits obviously different main sedimentary microfacies revealed by logging interpretation results, core facies, logging facies analysis and sandstone percentage maps. Taking into account of factors such as lake level and tectonic revolution, we propose a sedimentary evolution model for the Aradeiba Formation as follows. In the 2^nd early rifting stage, mainly distal bars and sandsheets deposited in the 5^th sublayer. As rift activity strengthened, with enriching sediment supplies, distributary channels and mouth bar developed in the 4^th and 3^rd sublayers, respectively, enlarging the shallow-water delta. Strengthening lake hydrodynamics at the 2^nd sublayer deposition resulted in sheetlike distributary channel dominance, while rapidly rising lake level led to deposition of mainly shallow lake drape mudstone in the 1^stsublayer. This study deepens the understanding of the sedimentology of the Muglad Basin and provides guidance for expanding hydrocarbon exploration in this basin.

Key words: shallow water delta, sedimentary characteristics, Aradeiba Formation, Unity Sag, Muglad Basin