Geochemistry of Crude Oils from Dulang Field: Some New Exploration and Development Insights into Source, Migration and Distribution of Hydrocarbons in the Malay Basin.

Abstract

Detailed geochemical data of oils from the Dulang field reveal the presence of three oil families, all of which come from very similar sources. The differences within the families reflect (1) fractionation processes operative during vertical migration and (2) minor contamination, especially by biomarkers extracted from coals by the migrating hydrocarbons. The deepest oils, which represent heavy residua from the fractionation process, are highly waxy, whereas the shallowest oils have low wax contents. The presence of free gas in association with the low-wax oils and its absence with the high-wax oils suggests that vertical migration may have occurred in the gas phase, with later separation of the light oil in the reservoir. Both associated and free gases contain a large amount of CO2, which is derived from a basement source.

The vertical and lateral distribution of oil and gas properties, in combination with other geological and geophysical data, suggest that hydrocarbon emplacement occurred along faults. In some cases, separate reservoir compartments can be identified on the basis of geochemical signatures of the oils. These conclusions have important implications for exploration and field development in the Malay Basin.

Introduction

The Dulang field contains in excess of one billion barrels of oil and a substantial amount of gas, making it one of the largest fields located offshore Peninsular Malaysia (Fig. 1). The faulted, anticlinal trap is some 25 km long by 3-4 km wide and contains stacked Miocene sandstone reservoirs interpreted to have been deposited in a tropical tidal to sub-tidal environment. At least two episodes of faulting and several significant stratigraphic pinchouts complicate the field geometry. A typical well log over the production section is shown in Fig. 2. The field has been under development from 1990 until late 1996, with data from 118 development wells from four platforms and 15 exploration/appraisal wells attesting to the large number of reservoir compartments. Producing Group D and E reservoirs in Dulang are for the most part normally pressured (1500-1850 psig) and lie between 1100 and 1500 m TVDss. Overpressure increases rapidly below approximately 1450 m, reaching in excess of 5000 psig at around 1850 m TVDss in the deeper Group F sands. Both undersaturated and saturated oils are produced, with a range of gas cap sizes. The oils are predominantly waxy (7-55%), with considerable variation in physical properties (Table 1). The gas contains very large amounts of CO2 (40-75%).

Geochemical techniques offer a useful, rapid and low cost set of tools to augment the more traditional geophysical, geological, petrophysical and engineering methods to learn more about vertical and lateral reservoir communication, migration pathways and gravity segregation within reservoirs. Such insights could have an impact on field depletion plans. The 1995 discovery of oil and gas in the overpressured Group F sands in Dulang stimulated further study to identify the source for those hydrocarbons and any relationships with the overlying oils. The results of the study reported here may lead to new exploration plays within Dulang as well as in other parts of the Malay Basin, particularly within the huge volume of untested overpressured section.

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