Diagenesis Model and Reservoir Characteristics within High Resolution Sequence Stratigraphic Framework in the Mawu-1 Submember in Ordos Basin,China

Key words: 
• Ordos basin /
• Mawu-1 Submember /
• high-resolution sequence stratigraphy /
• reservoir /
• diagenesis model

Abstract: Based on the theory of sequence stratigraphy, the sequence stratigraphic characteristics of Ordovician weathering residuals in central Ordos Basin were studied, and the high resolution sequence stratigraphic framework for the Mawu-l submember was established, in which 4 parasequences, i.e., Mawu1-1, Mawu1-2, Mawu1-3 and Mawu1-4 were identified. Generally, the parasequence sets of Mawu-l submember formed at the descent stage of sea-level, when the frequent change of sea-level resulted in multiple marines flooding surfaces and transgressive surfaces, and the deposits mainly consist of abnormal salinity possessing gypsum flat rocks. A parasequence may include one or more weathering-diagenesis cycles resulted from the change of sea-level. The top erosion surface of a parasequence corresponds to the maximum descent stage of sea-level. The relationship pattern between sea-level variation and weathing-diagenesis (karstification) was set up in this paper. The processes and controlling factors for inter-layer karstification and the weathering-diagenesis (karstification) at the later stage were presented from the genesis point of view. It was revealed that in carbonate high-resolution sequence stratigraphic framework, the sea level varying periods of different hierarchies controlled the sedimentary stratigraphy and weathering-diagenesis (karstification) of different hierarchies. Thus the multi-stage and multi-cycle characteristics of carbonate weathering residuals were determined. Therefore, the establishment of weathering-diagenesis pattern for parasequeces of Mawu1 submember is useful for further understanding of distribution pattern of weathering residuals, and for the effective prediction of weathering-crust reservoirs. The relationship between parasequences and reservoir characteristics were also analyzed. It was declared that reservoir characteristics of prapasequences 1 to 4 are integrated reflection of sedimentation and diagenesis. The influences of parasequence boundaries on the weathering-crust reservoir of Mawu-1 were also emphasized. These boundaries not only caused component and structure variations of rocks, but also provided materials for inter-layer karstification, since inter-layer karstification generally occurred at the top of parasequences, which exposed when the sea-level descended, and thereafter karstification took place. The vertical distribution of vugs caused by inter-layer karstification has obvious multi-cycles as primitive gypsum dotted sedimentation, while the horizontal distribution is controlled by sedimentary facies. In the study area, the accumulation capacity of parasequences 1 and 2 was influenced by karstific erosion at the later stage. Vugs in parasequence 3 are main hydrocarbon bearing spaces in the central gas field. Vugs in parasequence 4 are not main hydrocarbon bearing spaces for the limitation of karstification. Therefore, studies of sequence boundaries of different hierarchies are useful for the prediction of vertical and horizontal distributions of oil-gas reservoirs.