Curiale, J.A.¹, W. G. Dow²
(1) Unocal Corporation, Sugar Land, TX
(2) Consultant, The Woodlands, TX

ABSTRACT: Lacustrine-Sourced Oils of the Cuu Long Basin, Vietnam: Composition, Origin and Exploration Significance

Prolific lacustrine oil source rocks containing non-marine algal and terrigenous organic matter occur in the Cuu Long Basin, offshore Vietnam, and recently have generated and expelled oil into adjacent weathered, fractured basement and Miocene sandstone reservoirs. Physical properties and elemental concentrations of these oils, including density (25-37 API), sulfur, and vanadium + nickel contents vary considerably, but isotopic and source-specific molecular characteristics are very similar. Two oils, one from a fractured basement reservoir and another from a Miocene sandstone reservoir are compared and their source rock(s) characteristics defined.
Cuu Long Basin oils are generally pristine (not biodegraded or gas-stripped), show pristane/phytane ratios about two, and have little or no odd-carbon n-alkane preference. Land-plant molecular indicators, including oleananes and bicadinanes are accompanied by lacustrine algal indicators, including gammacerane and high concentrations of 4-methylsteranes and tetracyclic polyprenoids. These and other components indicate a mixed lacustrine organic facies containing both lipid-rich land plant debris and fresh water algal material. Because the source rock depositional setting was tropical, the fresh water lakes had no seasonal turnover and the depositional conditions resulted in reducing conditions and a stratified water column. This permitted the preservation of hydrogen-rich, oil-generating organic materials of both terrigenous and algal origin, resulting in highly prolific lacustrine oil source rocks.
Because some of these oils occur in basement rocks, they sometimes have been ascribed to an abiogenic origin, but their molecular composition totally refutes this possibility. Regional mapping of the source rock organic facies and thermal history in a petroleum system framework can help identify new prospective exploration trends in the Cuu Long basin.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.