The Seria Field was discovered in 1929. Cumulative oil production reached 164X10 6 m 3 (end 1996), approximately 34% of known in-place volumes. A maximum oil production level of 18 780 m 3 /d in 1956 has since declined to 2800 m 3 /d. Most of the more easily recoverable oil has now been produced. Undeveloped oil is concentrated in economically marginal accumulations for which simple subsurface models are an inadequate basis on which to plan further development activities. Instead, dynamic simulation of detailed computer-generated 3D reservoir models is required to optimize reservoir management and evaluate potential development options. 3D seismic data are used to assess undrilled closures and new exploration plays resulting in the development of new hydrocarbon accumulations. Amplitude analysis combined with GST/RST logging has identified areas of unswept oil within the field. Detailed 3D reservoir geological models integrating sequence stratigraphic concepts with reappraisal of core and wireline data are being built using Unix workstations. The models incorporate the results of advanced petrophysical techniques, such as image analysis and resistivity inversion, to quantify net sand, porosity and saturation, and NMR to provide information on moveable oil and permeability. The model forms the input to advanced reservoir simulators where multiple sensitivities can be tested to determine the optimum placement of new wells. Advances in drilling technology have led to the use of horizontal and multi-lateral wells to give the increased productivity necessary for commercial success of such marginal developments. Concurrent engineering effort has led to the field's facilities being rationalized to improve efficiency and reduce costs and the designing of re-usable well jackets for the shallow offshore part of Seria.

In a seemingly ever growing number of fields, the petrophysicist is being confronted by the challenge of evaluating thinly bedded formation sequences (i.e. beds whose thickness is below the resolution of most of the contractor "standard" open hole logging tools). The evaluation of such beds is indeed a challenge as more often than not the available wireline data does not provide anywhere near sufficient resolution to enable a proper evaluation of such beds. At best, the interval may be cored and indications of hydrocarbons detected. At worst, the interval could be totally over-looked due to misinterpretation and business opportunities missed. To assist the interpretation of such zones the mathematical technique of "inversion" has been applied to the resistivity suite of logs. Through the use of "inversion" or "resolution enhancement" software the apparent resistivities (derived directly from the resistivity tool) are automatically corrected for such environmental effects as shoulder-bed (bed boundary) effect, saline-borehole fluids and invasion. This results in "true" formation resistivity values being obtained on a layer by layer basis. The technique is not restricted to the more recent logging tools but is also applicable to the older resistivity tools (normal and lateral devices). This paper will demonstrate, through a number of field examples, how the inversion technique can be applied to substantially improve our current understanding of "true" formation resistivity and thus our "conventional" petrophysical evaluation. Introduction Over the years, a variety of techniques have been used by the petrophysicist in the evaluation of thin, hydrocarbon-bearing sand beds; here we take "thin" to mean beds whose thickness is in the range 5-60cm. Many of the deltaic sequences found in Brunei Darussalam exhibit good examples of thin hydrocarbon-bearing sands. Thus, it is essential that we have the ability to properly evaluate them. Quite often, however, this is not the case and reservoir characterisation is hampered by the lack of good quality logging data. The standard log-data-gathering techniques together with their modus operandi is given in figure 1. The integrated analysis of core with wireline data plays an extremely crucial role in the final interpretation. With good core data techniques are currently available to measure sand/shale ratio (net to gross), porosity and permeability. In contrast, however, true formation resistivity is solely reliant upon values obtained from resistivity log data. P. 123