The Effect Of Hydrocarbon Accumulation Upon Seismic Response Within The Barrow Sub-Basin, North West Shelf Of Australia: A Model-Based Study.
Honours Degree, 1993
University of Adelaide
A study of wells within the Barrow Sub-basin was undertaken to investigate the factors which may result in the presence of Direct Hydrocarbon Indicators (DHI) on seismic data. DHIs can take the form of a bright spot, dim spot or a polarity reversal on the reservoir reflection. Additionally, a gas-water interface may result in the production of a flat lying event. The observation of these types of events on seismic data can significantly reduce the risk involved with exploration drilling, but care must be take to ensure that they are, in fact, real events, rather than a product of other factors such as processing.
Sonic, density and acoustic impedance logs for representative water, oil and gas bearing wells in the Barrow Sub-basin were first cross-plotted to investigate the relationships between reservoir and seal impedances, and how these vary with hydrocarbon saturation and depth. This analysis also provided values for average velocities and densities for the modelling step.
Simple models were then constructed to investigate the effect of invasion on the acoustic parameters measured by well logs, and the effect of variable gas saturation on reservoir velocity. Another set of models, representing typical situations for various reservoirs and seals, was constructed. Synthetic seismograms were generated for these models, assuming water and hydrocarbon saturated reservoirs, in order to check for DHI effects.
It was found that the production of DHIs in the Barrow Sub-basin is most likely limited to the development of phase reversals and dim spots, due to hydrocarbon accumulation. Increased gas accumulation within the reservoir is not found to significantly effect the amplitude of the event noted at the seal-reservoir interface, indicating that DHI methods do not allow for the discrimination of commercial from non-commercial accumulations. The use of such simplified models does not necessarily allow the application of results to the basin as a whole, due to the wide lithological variation throughout the basin. More detailed studies are required, to firmly establish appropriate models for the Barrow Sub-basin. This should also include obtaining more confident estimates of the properties of hydrocarbon-bearing reservoirs in the presence of invasion effects on the logs.