RESERVOIR CHARACTERIZATION OF AN ISOLATED MIDDLE MIOCENE CARBONATE PLATFORM: CA VOI XANH FIELD, OFFSHORE VIETNAM

Abstract

The Ca Voi Xanh (CVX) gas field is located offshore Vietnam along the eastern margin of the southern Song Hong Basin. Reservoir rocks are carbonates of Middle Miocene (Langhian and Serravallian) age which developed on an isolated platform (length approximately 100 km, and width approximately 15 km) on top of the Triton Horst structural high. Shallow‐water corals and large and small benthic foraminifera are the main faunal constituents of the Langhian carbonates, whereas overlying Serravallian carbonates, the principal reservoir at CVX, are dominated by deeper‐water coralline red algae (rhodoliths) and large benthic foraminifera (LBF). The Serravallian carbonates consist of rhodolith‐LBF grainstones and packstones to mud‐lean packstones. Langhian carbonates consist of coral‐LBF grainstones‐packstone/rudstones.This paper documents the workflow used to develop an integrated sequence‐stratigraphic and reservoir rock‐type framework for the Ca Voi Xanh reservoir, and the impact of this work on reservoir quality prediction and modelling. Sequence stratigraphic interpretations of the Serravallian carbonates are based on data from three wells including detailed sedimentological core descriptions tied to well‐log character. The carbonate succession consists of three third‐order depositional sequences (Ser1, Ser2 and Ser3). Two well‐developed exposure surfaces can be identified: the base‐Serravallian sequence boundary (Ser1 SB), and the base‐Tortonian sequence boundary (Tor1 SB). The Serravallian shows an overall shallowing‐upward trend from carbonates with more horizontally‐oriented coralline red algae (encrusted and bored pavements/hardgrounds) in the lower part of the section, to carbonates with large, roundish irregular rhodoliths towards the upper part. Petrographic thin section, stable isotope (oxygen and carbon), and fluid inclusion analyses confirm a freshwater (vadose and phreatic) diagenetic overprint of the carbonates below the exposure surfaces (sequence boundaries).Partial dolomitization of the Serravallian carbonates was observed at Wells‐2 and ‐3 in the CaVoi Xanh field. Dolomite formation could be related to hydrodynamic fluid interactions associated with the development of a freshwater lense during the top‐Serravallian sea‐level fall (Tor1 SB), favoring dolomitization by sea water during the subsequent sea‐level rise. Alternatively, dolomitization could be related to CO2 and hydrocarbon charging, causing secondary leaching of the Mg‐rich coralline red algae and large benthic foraminifera, leading to an enrichment of the water with Mg‐ions and thus favoring downward dolomitization.A reservoir rock type (RRT) scheme was developed for the Langhian and Serravallian carbonates at the Ca Voi Xanh field based on a combination of depositional environment interpretations together with diagenetic characteristics and reservoir parameters (porosity and permeability).The Langhian is characterized by two RRTs depending on the degree of cementation (RRT‐3L) and dissolution (RRT‐5L). Serravallian RRTs are separated into dominantly packstone (RRT‐1) and dominantly grainstone textures (RRTs‐2, ‐3, ‐5 and ‐6). RRTs with grainstone texture show varying degrees of cementation (RRTs‐2 and ‐3) and dissolution (RRTs‐5 and ‐6). Early diagenetic dolomitization (RRT‐4) preferentially affects packstones (RRT‐1) but also grainstones (RRT‐2, ‐3 and‐5).The geologic (static) model consists of both matrix and non‐matrix components. Sequence stratigraphic concepts in combination with seismically‐derived palaeo‐reconstructions were used to guide the vertical and lateral distribution of reservoir rock types, which comprise the matrix component of the geological model. Different karst geometries and features consistent with known hydrogeologic processes were identified in the seismic discontinuity (variance) cube, and were used to interpret karst regions with different degrees (and/or types) of karst/fractures. These karst regions were used to populate the non‐matrix component of the geologic model along the Ser1 and Tor1 sequence boundaries.