Light Hydrocarbon Geochemistry of the Bonaparte Basin : Including the Sahul Syncline, Malita Graben and Northern Petrel Sub-basin : Rig Seismic Survey 99 : Project 121.24  

Authors / CoAuthors

Bickford, G.P. | Bishop, J.H. | O'Brien, G.W. | Heggie, D.T.

Abstract

During February-March 1991 the BMR conducted a combined Direct Hydrocarbon Detection (DHD) and high resolution seismic survey (Rig Seismic Survey 99; Fig. 1) in the Bonaparte Basin. This survey is one of three combined seismic and DHD surveys which were conducted in the Timor Sea (Fig. 2). Survey 99 collected approximately 3466 km of DHD, high resolution seismic, gravity, magnetics and side-scan sonar data. The data from this survey complement those obtained from Survey 100 (Bishop et al. 1992), which was conducted in the same general area. Three vibracores were taken in the northwestern corner of WA-217-P, while two grab sample sites were occupied in the vicinity of the Petrel gas field. Several bottom-water light hydrocarbon anomalies were detected during the survey, with most of these being located within the Petrel Sub-basin. A single, strong bottom-water anomaly, and several weaker hydrocarbon anomalies were found in the vicinity of the Petrel gas field, while weak, but aerally extensive anomalies were associated with the Tern gas field. No significant hydrocarbon anomalies were found in the Sahul Syncline. There were no strong C1 - C4 anomalies in the Malita Graben, however, one strong butane anomaly was found in the western end of the Malita Graben (principally composed of n-butane). Slightly elevated concentrations of light hydrocarbons were detected over an area of about 30 km in the eastern end of the Malita Graben (in the vicinity of the Heron 1 well). Cross-plots of percent hydrocarbon wetness versus methane indicate that the water column anomalies detected in the vicinity of the Tern and Petrel gas fields were sourced from a gas/condensate source, which is consistent with the known composition of the reservoired hydrocarbons at Petrel and Tern. The anomalies detected over the Tern and Petrel fields probably represent seepage directly from those accumulations. The general lack of hydrocarbon anomalies in both the Sahul Syncline and the Malita Graben may indicate that these source rock "kitchens" are no longer actively expelling hydrocarbons. A vertical profile conducted over one of the bottom-water anomalies near the Petrel gas field showed that the anomaly could be detected 40 metres above the seafloor. This highlights the fact that the DHD fish should be towed as close to the seafloor as possible if weak bottom-water anomalies are to be detected.

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14559

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• GA PublicationRecord

( Theme )

• geochemistry

( Theme )

• hydrocarbons

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• marine

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• Earth Sciences

• Published_External

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1992-01-01T00:00:00

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Record 1992/050

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