Reservoir characterisation of the Middle Miocene Ca Voi Xanh isolated carbonate platform

  • Christian J.Strohmenger
  • Lori Meyer
  • David S.Walley
  • Mazlina Md Yusoff
  • Donald Y.Lyons
  • Jacqueline Sutton
  • John M.Rivers
  • John M.Rivers
  • Beata von Schnurbein
  • Phong Nguyen Xuan
DOI: https://doi.org/10.25073/petrovietnam%20journal.v6i0.186
Keywords: Reservoir characterisation, carbonate platform, Ca Voi Xanh field

Abstract

The Ca Voi Xanh field is located offshore Vietnam in the southern Song Hong basin. Tertiary carbonates of Middle Miocene (Langhian and Serravallian) age form an isolated carbonate platform on top of the Triton Horst structural high. Shallow water corals and large and small benthic foraminifera are the main constituents of the older Langhian carbonates, whereas the overlying Serravallian carbonates are dominated by deeper water coralline red algae (rhodolith) and large benthic foraminifera (LBF).

Sequence stratigraphic interpretation is based on detailed sedimentolological core description tied to well-log character. A sequence stratigraphic framework was established for the Serravallian carbonates, displaying three third-order depositional sequences (Ser1, Ser2, and Ser3).

Langhian carbonates can be described by one lithofacies: coral-LBF-foraminifera grainstone-packstone/rudstone. Depending on the presence of mud in the samples, two lithofacies types are characteristic for the Serravallian: LBF-rhodolith packstone to mud-lean packstone and rhodolith-LBF grainstone to mud-lean packstone.

Two well-developed exposure surfaces can be identified on top of the Langhian (Serravallian sequence boundary Ser1_SB) and on top of the Serravallian (Tortonian sequence boundary Tor1_SB). Serravallian carbonates show an overall shallowing-upward trend from more horizontally-oriented coralline red algae (encrusted and bored pavements/hardgrounds) at the lower part of the section to large, roundish, irregular rhodoliths towards the upper part of the section. 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 dolomitisation observed at Well-2 and Well-3 might be related to CO2 and hydrocarbon charging, causing secondary leaching of the Mg-rich coralline red algae and LBFs; leading to an enrichment of the water with Mg-ions and thus favouring downward dolomitisation of the Serravallian carbonates.

A reproducible reservoir rock type (RRT) scheme was developed for the described carbonates, using a combination of depositional environment, diagenetic overprint, and reservoir parameters (porosity and permeability). The Langhian is characterised by two RRTs, depending on the degree of cementation (RRT-3L) and dissolution (RRT-5L). The Serravallian RRTs are separated into dominantly packstone (RRT1) and dominantly grainstone textures (RRT-2, RRT-3, RRT-5, and RRT-6). RRTs with grainstone texture show varying degrees of cementation (RRT-2 and RRT-3) and dissolution (RRT-5 and RRT-6). Early diagenetic dolomitisation (RRT-4) preferentially effects packstone (RRT-1), but also grainstone textures (RRT-3 and RRT-5). The vertical and lateral distribution of RRTs, supported by seismically-derived paleo-reconstruction of the carbonate platform, adequately describes the reservoir.

The geologic (static) model consists of both, matrix and non-matrix components. Seismically-derived probablitiy maps for each of the six Seravallian RRTs, based on 3D seismic paleo-structure mapping, were used as input to the matrix component of the geologic model. Different karst geometries and features consistent with known hydro-geologic processes are identified in the seismic discontinuity (variance) cube and are used to interpret karst regions with different degrees and/or types of karst/fractures. These karst regions are used to populate the nonmatrix component of the geologic model along the identified two major sequence boundaries (Ser1_SB and Tor1_SB).

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Published
2018-06-30
How to Cite
J.Strohmenger, C., Meyer, L., S.Walley, D., Md Yusoff, M., Y.Lyons, D., Sutton, J., M.Rivers , J., M.Rivers , J., von Schnurbein, B., & Nguyen Xuan, P. (2018). Reservoir characterisation of the Middle Miocene Ca Voi Xanh isolated carbonate platform. Petrovietnam Journal, 6, 10-24. https://doi.org/10.25073/petrovietnam journal.v6i0.186
Issue
Vol 6 (2018)
Section
Articles

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