Sequence-Stratigraphic and Facies Controls on Reservoir Quality of Early to Middle Miocene, Formation2, Gulf of Thailand

Abstract

Summary Formation 2 is the main contributor to hydrocarbon production in the Gulf of Thailand. Nine key lithofacies were deposited in fluvial and tide-dominated deltaic environments. Two methods of electrofacies classification were used to estimate rock types in non-cored wells, including Artificial-Neural Networks and K-means clustering. For mapping purposes, lithofacies are combined into four lithologies: 1) coal, 2) claystone, 3) heterolithic sandstone, and 4) sandstone. Using ANNs classification with an overall accuracy of 85%, lithology logs were estimated to establish a sequence-stratigraphic framework and to map reservoir properties. Formation 2 stratigraphic framework consists of five third-order cycles named units 2A-E. The moderate eustatic sea-level rise approximately 19 Ma resulted in a variety of depositional environments, facies distributions, and reservoir properties. Units 2A-C represent a continuous transgression and landward shift of facies. The top of unit 2C possibly indicates the maximum landward extent of shoreline. Unit 2D records a major regression resulting from a glacio-eustatic sea-level fall and tectonic uplift in this region. Three-dimensional models illustrate the distribution of lithology, porosity, and permeability of fluvial and tide-dominated deltaic deposits. Sandstone percentage and reservoir quality relates to the regressive cycle, unit 2D, while transgressive cycles 2A-C exhibit lower sandstone content and reservoir quality.