Abstract

A calcareous Miocene shale separates the "Miocene" and "Plio-Pleistocene" plays in the eastern Andaman Sea and is the principal hydrocarbon source rock in this area. Three Miocene play gas accumulations encountered have 10-90% CO₂. The application of diverse geochemical and petrographic techniques was used to establish the origin and rationalize the distribution of CO₂ in the Miocene play and, considering this information within the geological framework, quantify CO₂ risk in the Plio-Pleistocene prospects.

Gas compositional and isotopic analyses indicate a "crustal" (³He/⁴He) and principally "inorganic" (δ¹³C > -10) origin for the Miocene CO₂. Artificial maturation of the Miocene shale yields methane and CO₂ with stable carbon isotope values similar to those of the high CO₂ Miocene gases. This and the presence of multiple generations of calcite cements in the underlying Miocene carrier/reservoir system point to CO₂ release from diagenetic cements.

The abundance of CO₂ among the Miocene reservoirs appears to be a function of migrant fluid chemistry and temperature. This is in turn controlled by structural timing and configuration. Fluid pulses were identified using provenance indicators (Sr isotopes and minor elements from residual brines and early and late calcite cements) and fluid inclusions. These include connate marine water expressed from ~40-15 Ma clastic marine sediments and recent fluids derived from hydrothermal exposure. Oil initially emplaced in the high CO₂ Miocene reservoir was mostly replaced by CO₂ mobilized by the hydrothermal event. The hydrocarbon-rich reservoir was largely shielded from hydrothermal fluids by a down dip fault bordering with the intermediate CO₂-rich reservoir.

The prognosis is good for low CO₂ gas in Plio-Pleistocene prospects because (1) the CO₂ flux (indicated by gas analysis of crushed cuttings) is lower than the Miocene system and (2) the concentration of hydrocarbons relative to CO₂ is likely enhanced by the higher solubility of CO₂ and by generation of biogenic methane. Subsequent drilling showed that the Plio-Pleistocene gas accumulation is indeed lean in CO₂.