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.