Petroleum Geoscience - Volume 10, Issue 3, 2004

This paper elaborates a methodology for assessing the consequences of uncertainties in the source and reservoir models on hydrocarbon phase distributions in prospects by improving the a-priori probability distributions. The uncertainties of geological systems can be described by probabilistic distributions of quantified geological variables, such as source rock and reservoir thicknesses. A Monte Carlo simulation of hydrocarbon generation, expulsion, migration and entrapment is carried out with probabilistic distributions defining important input variables. A weight for each simulation run is computed from a misfit function that describes how well the modelled oil and gas columns fit the observed columns in a set of calibration wells. These weights are used to compile a-posteriori probability distributions of the input variables. The resulting probability distributions, therefore, describe the uncertainties of the geological system after the calibration of the migration model.

In a demonstration example from the Tampen Spur area of the North Sea, the most sensitive geological input distributions become significantly narrower, thus limiting the range of possible outcomes. Other distributions become more uneven, with peaks and lows. The changes in the variable distributions have been quantified and variables can, therefore, be ranked with respect to their impact on changes in probabilities for key outcomes. A large number of simulation runs is required for the proposed method to be effective because many simulation runs result in too large a misfit. A statistical fit of a second-order function to the misfit values allows for the effective screening of input variables, thus reducing the computing time, in the demonstration example, to 15% of the unscreened times. The result of the analysis is that the effect of geological uncertainties on output values is effectively reduced and that a quantitative learning has been obtained. This knowledge can be applied in other hydrocarbon generation and migration studies within the study area, allowing for more rapid and more accurate estimates of oil and gas resources before drilling.

A coupled pressure and stress simulator has been applied to the reservoir sandstone unit, the Middle Jurassic Garn Formation in the Halten Terrace, offshore Mid-Norway, in order to simulate hydraulic fracturing and leakage due to overpressure. The over- and underlying rocks are used as vertical seals to the compartments. The overpressure generation within the pressure compartments was modelled quantifying the mechanical and chemical compaction. An empirical model for the minimum horizontal stress was applied to the Griffith–Coulomb failure criterion to estimate the pressure levels at which hydraulic fracturing occurs.

Sensitivity tests of Poisson's ratio and Young's modulus, which control Biot's constant, show an effect on the accumulations of overpressure during time. Defining Biot's constant equal to 1 results in too early pressure build up and hydraulic leakage in the western parts of the basin. Lower values of Biot's constant (>0.85) give a present-day pressure distribution closer to the measured pressures in wells. The Kristin compartment is simulated to experience hydraulic failure around 1.5 Ma. High values of the coefficient of internal friction (μ) and the coefficient of sliding friction (μ′`), which are used in the failure criteria, results in a time-delay in failure and, thereby, less leakage from overpressured compartments.

The purpose of this study was to examine the effect of grain contacts and quartz cementation with regard to acoustic wave propagation in sandstones. Grain contacts have been considered essential when relating acoustic velocities to physical rock properties, and a parameter numerically representing the contact area between individual grains (contact length) has been measured. The method used involves digital petrographic microscopy pictures analysed by image analysis software. Other parameters, such as grain size, number of grain-to-grain contacts, cracks, clay content and porosity, have also been closely examined. The results showed that the contact area between sand grains may be useful for explaining velocity variations during the initial stages of quartz cementation and grain framework stiffening. Continued increase in grain-contact area by chemical compaction will have less influence on the sonic velocity compared to variations in porosity. The Garn Fm. from the Norne Field (2.6–2.7 km burial depth), which is in the early phase of initial grain framework stiffening by quartz cementation, and the Garn Fm. from the Kristin Field (4.6–4.7 km burial depth), which is thoroughly quartz cemented, were chosen as sample materials. Log-derived velocities, in addition to some laboratory ultrasonic velocity measurements, were used in this study.

This paper describes the distribution, seismic characteristics and potential volume of gas hydrates in the eastern part of the North Makassar Basin, Indonesia. Identification of the gas hydrates depends entirely on the presence of extensive bottom simulating reflectors (BSR) which are distributed primarily within the offshore extension of the West Sulawesi Fold Belt. Turbidite deposition within the fold belt mini-basins provides a reservoir and a source of organic material for the production of biogenic methane gas. Several seismic examples are shown, including a suspected palaeo-BSR on the crest of an eroded anticline, a submarine mud volcano and an occurrence of rotated slump blocks and debris flows that glide along the hydrate/free gas interface.

Geothermal gradients derived from the BSR database averaged 4.7°C/100 m, whilst derived heat flow values varied from 20 mW m−2 to 60 mW m−2, comparing favourably with regional heat flow data. Gas-in-place estimations for the most contiguous accumulation of gas hydrates (c. 8000 km2) yielded an estimated 1.89×1012 m3 (67×1012 SCF) of methane gas. Free gas in place below the hydrate zone within the same area was estimated to be 379×109 m3 (13×1012 SCF).

A method for correcting log-derived temperatures in deep wells (3500–6500 m) has been developed by comparing log temperatures from the Gulf of Campeche (Mexican Gulf of Mexico) with DST temperatures in the same wells. The equations developed in this study are modified slightly from those of Waples & Mahadir Ramly (2001) , which were calibrated using data from depths <3500 m in Malaysia. The correction depends strongly on time since end of mud circulation (TSC) and, to a much lesser degree, on depth. In this study the true subsurface temperature (Celsius) is given by

An estimate of the uncertainty in the corrected temperature is obtained from the equation:

Porosity–effective stress analysis of 37 wells in the Carnarvon Basin, Australia, has identified 12 wells where the overpressure was generated by disequilibrium compaction and four wells where the overpressure was generated by fluid expansion. Disequilibrium compaction was identified as the dominant overpressure-generating mechanism in wells along the Rankin Trend as far south as Gorgon 1 and Spar 1. Fluid expansion was identified as the dominant mechanism of overpressure generation in wells along the Barrow Trend and around the Alpha Arch. Disequilibrium compaction-generated overpressures occur, as would be expected, where the Tertiary sediment thickness is greatest and fluid expansion overpressures where the Tertiary is thinnest. Indeed, where the N-1 (35 Ma) reflector is greater than c. 1500 m below seabed, disequilibrium compaction overpressures are observed and where itis shallower than c. 1500 m, fluid expansion overpressures are observed. Acoustic log-based pore pressure detection using Eaton's (1975) method yielded accurate estimates with an exponent of three in Wilcox 1 where the overpressure was generated by disequilibrium compaction, and an exponent of six in Bambra 1 where the overpressure was generated by fluid expansion. If the Eaton (1975) technique is to be applied to seismic processing velocities for pore pressure prediction, then an exponent of three can be used where the N-1 horizon is greater than c. 1500 m and an exponent of six where the N-1 horizon is less than c. 1500 m.

The paper presents the results of laboratory compaction tests on two samples of the lower Kimmeridge Clay Formation obtained from two different outcrop localities in the UK. The purpose of the testing programme is to study the compaction behaviour of an argillaceous sediment from the same parent material but with different diagenetic histories. Since most available data are for clays subjected to moderate effective stresses (less than 20 MPa), the compaction laboratory tests were carried out to as high as 120 MPa, corresponding to about 9 km burial. The microstructure of the tested materials was studied in a scanning electron microscope before and after compaction testing to evaluate the effects of diagenesis on the microstructure of argillaceous sediments. The two tested materials have, in the past, been subjected to different burial depths and degrees of chemical precipitationand cementation. Several pieces of evidence are shown of the effects of mechanical and chemical processes on the compaction behaviour of argillaceous materials. Mechanical compaction can only account for part of the porosity loss and increase in stiffness of argillaceous materials during burial. Mechanical compaction produces an anisotropic rearrangement of clay particles in uncemented sediments. Once chemical diagenesis has taken place, chemical cementation preserves the microstructure of the sediment and only minor changes in clay particle arrangements take place during further mechanical compaction, even when very high stresses are used. Also, it is seen that chemical processes reduce the permeability more strongly than mechanical compaction.

Compositional changes amongst surface active components of petroleum are brought about by crude oil/brine/rock interactions. The behaviour of petroleum components following simulated petroleum migration through an initially water-saturated continuous siltstone (89.7 cm × 5 cm) core was investigated. A suite of produced oils and core extract petroleum from the core-flood experiment were analysed by Iatroscan and Gas Chromatography-Mass Spectrometry (GC-MS). Dramatic changes were observed in the composition and distributions of polar non-hydrocarbons, namely, fluoren-9-ones, carbazoles, benzocarbazoles and C0–C3 phenols indicating varying degrees of surface activity within the core-flood system.

Following flooding, environmental scanning electron microscopy (ESEM) was used to view water condensation behaviour on freshly exposed core chips. Samples prepared from the original unflooded core and core material sectioned towards the outlet of the core flood displayed surfaces exhibiting hydrophilic tendencies. Meanwhile, hydrophobic behaviour was observed in a sample from the core inlet. The ESEM descriptions of the interaction of water on core surfaces appear to be consistent with the compositional changes encountered in the core extract petroleum. Wettability was altered most strongly at the core inlet where surface active compounds are rapidly removed from the migrating petroleum. In particular, small hydrophilic moieties, such as alkylphenols, appear to rapidly precondition mineral surfaces, changing wettability and subsequently allowing larger hydrophobicmolecules to sorb. Hence, petroleum surface active compounds are capable of rapidly (days to months) facilitating wettability alteration in crude oil/brine/rock systems that are initially water wet.