Jiyuan region is located in the western Ordos Basin, which belongs to Yishan slope of Ordos Basin. Its sedimentary facies are controlled by provenance from northeast, northwest and south directions in late Triassic. A shallow... [ view full abstract ]
Jiyuan region is located in the western Ordos Basin, which belongs to Yishan slope of Ordos Basin. Its sedimentary facies are controlled by provenance from northeast, northwest and south directions in late Triassic. A shallow water delta depositional system was developed in this period. Based on a great number of core observations, well log data, laboratory test and characteristics of modern shallow water lake sediments, combined with basic principles of high-resolution sequence stratigraphy, we made the following conclusions:
1) The Lithology of Chang 82 Oil Group are mainly fine lithic feldspathic sandstone and feldspathic detritus sandstone. The content of feldspar is 25%－40%, average in 31.25%, quartz accounts for 16%－46%, average in 28.54% and debris is 20%－35%, average in 28.31%. The grain of the sandstone was fine, sub-angular, but sorting medium-good. In sum, sandstone of the study area had low compositional maturity but middle textural maturity.
2) Delta-front subfacies mainly developed in the study area. Sedimentary microfacies are consisted of underwater distributary channel, subaqueous levee and tributary bay. The mouth bar and the gravity flow deposits are not well developed, The river has been branched off or diverted frequently due to the lake water obstruction and are widely distributed to a long distance with larger scales.
3) Chang 82 Oil Group can be divided into four short-cycles in ascending order(SC1, SC2, SC3 and SC4), Based on analyzing the relationship among the cyclic stacking pattern of various grade of base-level and the distribution and evolution of sedimentary facies and compared with the normal delta, a high-resolution sequence stratigraghic frame-work and stratigraphic-lithofacies palaeogeographic maps of SC1, SC2, SC3 and SC4 are established by taking the short-term cyclic sequence as a unit. Compared with the normal delta, the shallow-water delta is characterized by its low A/S, no mouth bar and the microfacies where favorable reservoirs developed is the sandbodies of distributary channel which extended long and mainly developed in the period of the rising half cyclic sequences of short-term.
4) The Chang 82 Oil Group is formed under the conditions of proximal material source,high-energy and sufficient material supply. Combined with characateristics of modern shallow water lake sediments, the paper concluded the depositional model is the distributary channel deltas when the channel energy is greater than the jacking force of lake water upon a river entering a lake, and the river channel continues extending forward to form banded, vertical-stacking thick sandbodies.
Topics: Deltaic depositional systems , Topics: Lacustrine sedimentation , Topics: Sequence stratigraphy
