WATER WASHING IN THE NORTHERN BONAPARTE BASIN

Abstract

The discovery of 11 oil fields and one gas field in the Northern Bonaparte Basin since 1994 has established a new petroleum province. The prolific yield of the Middle to Upper Jurassic source rocks is demonstrated not only by the volumes of reservoired hydrocarbons, principally in the Plover and Elang formations, but by the long residual columns beneath a number of the fields, and in some dry structures. An important aspect of the continuing exploration in the basin is, therefore, to identify prospects where as much as possible of the hydrocarbon column is preserved. While the integrity of fault seals has, until now, been the primary focus in this regard, this paper proposes water washing as the principal mechanism for depletion of hydrocarbon accumulations within the Northern Bonaparte Basin.

That such a process might have operated was indicated initially by the observation that, while the oils in the basin are so light that they are almost condensates, they are also extremely low in volatile content or, in other words, undersaturated. This phenomenon strongly suggests selective removal of compounds. The identification of this process as water washing was based on the relationship between the light aromatic content of the oils, and their gas-oil ratios (GOR) and bubble-points. Within the oils characterised by very low GORs, highly soluble light aromatics, such as benzene and toluene, are almost completely absent, whereas under conditions of evaporative fractionation by fault leakage these compounds tend to be enriched in the residual oil. The fact that methane, ethane and propane are also highly soluble, and have therefore also been removed, accounts for the low volatility of the oils. The lightness of the original hydrocarbons has probably disguised the process of water washing, as only the very soluble components have been removed.

The volume loss, under reservoir conditions, resulting from the depletion of a Northern Bonaparte Basin oil accumulation by water washing has been calculated to be in the order of 70%. The volume loss of degrading a gas/condensate accumulation to a low GOR oil is around 90%. These volumetric losses are consistent with the dimensions of many of the residual columns observed in traps in the area.

Regionally, the degree of water washing increases to the northwest, with fields such as Laminaria and Buffalo having the lowest light aromatic content. Offset pressure data from reservoirs indicates a present-day water flow from the northwest. This flow can be accounted for by the dewatering of sediments overthrust by the island of Timor over the last seven million years.

Compositional variation of light molecular weight compounds, within some fields, may also be attributable to water washing, with reservoir heterogeneity hindering the diffusion and homogenisation of hydrocarbons through the fields. These compositional variations strongly indicate that water washing is occurring at the present-day, and consequently may be of value in reservoir production studies. Hydrocarbons recovered from the Darwin Formation, which is not in communication with the Elang/Plover aquifer, exhibit little or no evidence of water washing.

The proposal that water washing can remove significant volumes of hydrocarbons from traps does not appear to have been previously documented. It constitutes a significant advance in our understanding of exploration risk in the Northern Bonaparte Basin by demonstrating that small isolated closures or deep crests within regional highs carry a significant risk of being underfilled. Moreover, column height within prospects may be estimated by calculating volume losses from fields 'along strike' in regard to the degree of water washing.

A strong incentive to explore for an alternative play type is provided by the recognition of non-degraded oil within traps not in communication with the Elang/Plover aquifer.