Source rocks in sequence 13A (Early Aptian)

Source rocks in the 13A sequence are developed across most of the basin (Davies, 1988b and Brink et al., 1991). The considerable thickness and extent of this source and its significantly higher richness than any of the other source rocks make it highly prospective. The Early Aptian source is considered to have been formed in an anoxic basin in which organic matter deposited in the core of the basin was dominated by oil-prone material and was rimmed by gas-prone sourec rocks. There is a small area near the basin centre where no source rocks have been found (between wells 124 and 160). This dows not negate the anoxic basin model as it is though that this was a high during deposition of Lower Aptian rocks, possibly a relict of an earlier inversion (perhaps during Hauterivian), when the sea floor may have been above the oxygen minimum.

92 Support for this comes from the thinly bedded nature of the sands in over and underlying sequences, evidence of a topographic high lasting a considerable time. Indeed, this region is still a relative high in post-13A, based on the bifurcation of the 14A sandstone-rich trend around it (Hill, 1991). In well123 at the western end of this high there are thin intervals of source rock indication that potential existed at least intermittently for source rock deposition. This suggests that the 13A source rock was deposited in the “silled basin” model probably silled by the early eastern highs. The 13A sequence is seen to blanket all the highs in the eastern part of the basin and it is therefore likely that the source rock was deposited by hemi-pelagic sedimentation (Benson, J.M,. 1993, pers.comm.).

In the northern flank of the basin, in spite of the considrable areal extent of wet gas-prone source rocks, the maturation level is generally too low to result in significant gas generation, except in the eastern region were sediments have been matured into the gas window by the instrusions. The buld of the oil-prone material is found to the south of the present day basin axis. As with other source rock intervals, HI vs Tmax shows all samples located in the Type 2 region, but towards the upper side in contrast with all other intervals. This indicates a greater oil potential in this interval than in any other. Average TOC’s are significantly higher than in other intervals, ranging up to 4.0% with HI’s ranging up to >500. Generation potential exceed 12 kg/tonne at the present day. Extrapolation along the trend shown in Davies (1990) suggests that original potentials could have been in excess of 14 kg/tonne rock.

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6 CHAPTER

CORE ANALYSIS AND INTERPRETATION OF BOREHOLE LOGS

The core analysis is done to establish ground truth for other formation evaluation measurements and is essential for calibration of well logs. Core analysis is a tool in reservoir assessment that directly measures many important formation properties. The objective of performing this analysis is to bring a sample of the formation and its pore fluids to the surface in an unaltered state to preserve the sample and then transport it to the laboratory for analysis. The analysis may aim to determine porosity, permeability, fluid saturation, grain size distribution, mineral composition, grain density; etc. samples for this analysis may come from conventional core, sidewall cores or plugs, and cuttings (Bateman, 1985). The core analysis is usually carried out on core plugs, samples that are taken from the bulk core. In the core laboratory, core plugs are drilled from whole core that typically have a length of about 5cm in diameter of 2.5cm. The petrophysical properties are then measured on these core plugs. Laboratory core analysis can provide very accurate measurements and are regarded as the ground truth.

Porosity determinations in the laboratory are accurate within ±0.5% of the porosity value and ±5% of permeability when the limits and procedures are properly observed. Samples of core taken with either water or oil base mud and are preserved and subsequently tested without cleaning and drying are referred to as fresh cores. Sample of cores cleaned and dried prior to testing are referred to as restored core. An advantage is that air permeability and porosity are available to assist in sample selection (Core laboratories, 1973). The special core analysis (SCAL) are measurements that are made on core plugs that complement the routine core analysis measurements which provides information on the electrical properties, relative permeability, capillary pressure, cation exchange capacity (CEC) and wettability. The results of electrical properties of rock measured from SCAL analysis include the resistivity formation factor, cementation exponent, resistivity index, and also the determination of saturation exponent. The results of the relative permeability measurements helps to make quantitative estimates of formation damage, quantifies effective permeabilities of water, oil and gas, and calculate cumulative permeabilities to each different fluid.

94 The common goal in the oil industry is to maximize the net revenue from the hydrocarbon- bearing resources that we discover and develop. This can be summarized in the following equation:

Profit=f(A*H*N/G*Ø*Sh*R)

Where: A*H = Gross rock volume, N/G = Net to gross ratio, Ø = Porosity,

Sh = Hydrocarbon Saturation,

R = Recovery factor.

This equation illustrates that the economic result is a function of the volume of hydrocarbons in place in the reservoir at any time. Log analysis concentrates on quantifying the petrophysical parameters such as G/G, Ø, Sh and input to R.