Abstract

The high cost of offshore exploration has made search for, and the identification of, sea floor seeps a well-accepted risk assessment methodology. Oil and gas seeps in near-surface seafloor sediments can provide evidence of active oil generation/migration and allow better assessment of prospective areas. In addition, samples are often available for the characterization of oil properties, maturity and source rock type. Also, the spatial coincidence of seepage and geologic structure allows for the identification of the loci of natural hydrocarbon seepage and to infer possible migration pathways. The authors have been involved in the collection and analysis of nearly 15,000 piston cores over the last twenty years as part of such deepwater surface geochemical exploration (SGE) studies in frontier regions worldwide, and present here the results of such a study offshore Borneo.

Analytical measurements of a surface geochemical survey include the basic measurements on every sample of: (1) an aromatics screen by total scanning fluorescence intensities (TSF Max Int.) of sediment extracts, (2) the C15+ hydrocarbons, by gas chromatography, in the sediment extracts, and (3) the interstitial light hydrocarbon gases in separately canned sediment sections by headspace extraction and gas chromatography. C15+ hydrocarbon analysis also includes determination of the isoprenoids pristane and phytane, and a determination of the unresolved complex mixture (UCM). From the analyses, these screening indicators of migrated petroleum hydrocarbons are used to qualify cores as having evidence of thermogenic hydrocarbon seepage and distinguish them from background for a study area. Samples thus shown to contain migrated petroleum are analyzed for saturate and aromatic biological markers and stable carbon isotopes. These more detailed analyses can confirm the presence of migrated petroleum and are used to match hydrocarbons in the sediment extract with a specific produced oil and/or source rock. Results of biological marker determinations in seep samples are compared to results of such analysis on oil samples from the region.

For these purposes, ten piston cores were acquired for Amerada Hess in its Block F offshore Malaysia in 2002. Geochemical screening identified thermogenic gas seepage in six of the cores, and oil seepage in three of these six cores. Screening parameters for both thermogenic gas and oil were applied to all samples of the program. In these plots, background values of the indicators plot below a threshold line and anomalous, seepage-indicating values plot above the line. The samples that showed indications of thermogenic gas were then analyzed for the carbon isotopes in each of the light hydrocarbon gas components to further understand the source of the gas. The samples that showed indications of oil seepage were then analyzed for biological markers to further understand the source of the oil. The carbon isotopes of the gases confirmed the presence of thermogenic gas. The biological markers revealed a considerable amount of detailed information about the oil seepage.

Extracts from two selected piston core sections were subjected to liquid chromatography in order to isolate the seep oil saturate and aromatic hydrocarbon fractions for subsequent stable carbon isotopic and biological marker analyses by Gas Chromatography/Mass Spectrometry. Both of these sections contained the same migrated crude oil from thermogenic sources in addition to some Recent immature organic matter (Rom). The seep oil was geochemically assessed and compared to a few subsurface oils from the region for the purpose of predicting source rock type, age, and thermal maturity for the seep oil.

Surprisingly, the seep had not suffered biodegradation (i.e., loss of n-paraffins), suggesting active seepage. With the exception of the steranes, it appears that relatively minor immature terpenoids from Recent organic matter (Rom) are 'contaminating' the signal from thermogenic terpanes. The steranes appear to be mostly Rom. The terpane distribution is highly suggestive of paralic marine shale/deltaic coaly source rocks containing mostly gas-prone type III kerogen with some oil-prone resinite. The abundant C19 and C20 tricyclic terpanes, as well as the presence of bicadinanes from resinous coals is paramount. Also, the significant oleanane component and des-a-oleanane, as well as other flowering plant biomarkers, suggest a Tertiary age for the source. The triaromatic steranes suggest a low to moderate level of thermal maturity and do not appear to be adversely affected by Rom. The terpanes from the seep correlate better with oil from Jintan-1 than with samples from the D-Alpha field, although the stable carbon isotopic composition is more compatible with the latter.

The views and interpretations presented in the following paper are that of TDI-Brooks International, and not necessarily of block F operator Amerada Hess or any of its Partners.

Presented at: 2005 South East Asia Petroleum Exploration Society (SEAPEX) Conference, Singapore, 2005