--> Stratigraphic sequence and Facies Control on Reservoir Quality and Productivity of Early to Middle Miocene Fluvial-Deltaic Deposits, Formation 2, Gulf of Thailand

AAPG ACE 2018

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Stratigraphic sequence and Facies Control on Reservoir Quality and Productivity of Early to Middle Miocene Fluvial-Deltaic Deposits, Formation 2, Gulf of Thailand

Abstract

The Early to Middle Miocene Formation 2 is a main contributor to the hydrocarbon production in the Gulf of Thailand. The Formation 2 consists of nine lithofacies that were deposited in coastal-plain and marginal-marine environments and include 1) coal, 2) organic claystone, 3) bioturbated and laminated claystone, 4) bioturbated and laminated heterolithic sandstone, 5) structureless sandstone, 6) cross-bedding sandstone, 7) parallel laminated to ripple cross-laminated sandstone, 8) sandstone interbedded with claystone, and 9) conglomerate. To estimate lithofacies in non-cored wells, two methods of electrofacies classification are used including an artificial-neural network (ANN) and k-means clustering analysis. However, because lithofacies prediction resulted in a relatively low overall accuracy (~60%), for mapping purposes, the lithofacies were combined into four lithologies: 1) coal, 2) claystone, 3) heterolithic sandstone, and 4) sandstone. Using the classification methods, lithology logs were estimated for all wells with an overall accuracy of 80% and used for stratigraphic correlation with other well logs.

Formation 2 strata forms a subset of a transgressive systems tract that includes the underlying Formation 0, Formation 1, and the overlying Formation 3. The Formation 2 stratigraphic framework consists of a single parasequence that represents the top of an upward-fining retrogradational parasequence set and is overlain by 4 parasequences of a coarsening-upward progradational parasequence set. The parasequences form five major stratigraphic zones named, from deeper to shallower, units 2A, 2B, 2C, 2D, and 2E. The unit 2A mainly consists of fluvial-deltaic heterolithic sandstones and claystones, and units 2B-2E are composed of deltaic-dominated sandstones and heterolithic sandstones.

Three-dimensional reservoir models that are constrained to core, well logs from 105 wells, estimated lithology logs, and 3-D seismic data illustrate the spatial distribution of lithologies, faults, porosity, and permeability of the fluvial-deltaic deposits. In general, higher porosity and permeability directly relate to upward-coarsening parasequences. Hydrocarbon production is controlled by a combination of the stratigraphic variability of fluvial and deltaic sandstones and fault compartmentalization.