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A Qualitative Flow Model for Parallel Bedding and Simple Cross Beddings Sandstones to Predict Reservoir Quality

Abstract

This paper aims to research the impact of different depositional texture types on reservoir quality and oiliness. We studied the First Subsection of Funing formation in Gaoyou Sag, Subei Basin and the Chang8 subsection of Yanchang formation in Longdong area of Ordos Basin, both of which are tight sandstone reservoirs characterized as shallow water deltas. These two reservoirs have a plentiful of bedding types and a heterogeneity in oil-bearingness. Base on cores from 38 wells in Gaoyou Sag and 54 wells in Ordos Basin, we summarize three general rules: 1) in none-massive bedding cores, oily parts extension corresponds with that of bedding series interfaces. 2) the carbonate cementation belt extends in the direction of the bedding series interfaces. 3) in a given depositional series, cores with massive beddings are the most probable to be oil-bearing, then parallel beddings take second place, the cross beddings are the least probable. Thin slices and casting thin slices are used to observe the initial ingredients and diagenesis of different parts in a given series of bedding. We find that in a given parallel bedding series or a cross bedding one, the average point granularity correlates with the range of which the point is to the series interface. However, in massive bedding sandstones, the average granularity distributes more uniformly. In different bedding types of sandstones, original deposits anisotropism differs with bedding types. Reservoir quality is initially controlled by deposits anisotropism. Different parts of a bedding series play differential role for fluids-- the finer part may be flow barrier while the coarser part may be preponderant flow path. In the interaction of reservoir pores and formation fluids, the role of different parts in a bedding series may change. The change corresponds with diagenesis or oil injection and reservoir quality anisotropism. This process is simpler in typical parallel bedding or cross beddings (climbing ripple bedding, tabular cross bedding and wedge cross bedding). It will be more complicated in transitional beddings. Using water drive experiment of sandstone samples and point porosity and permeability measurement method, a qualitative flow model for parallel bedding and simple cross beddings sandstones is proposed to simulate the process of interaction of reservoir quality and formation fluids in diagenetic evolution process.