Iain C. Scotchman

The effect of water pressure on hydrocarbon generation reactions: some inferences from laboratory experiments

ABSTRACT For the last twenty-five years most petroleum geochemists and basin modellers have produced and used models for maturation and hydrocarbon generation reactions in geological basins that do not consider pressure as a primary control. These conclusions are based on extensive laboratory investigations mainly using pyrolysis. Chemical theory, however, indicates that endothermic volume expansion reactions, such as maturation and hydrocarbon generation, are controlled by both the system pressure and temperature, and geochemists and basin modellers may need to reconsider the importance of pressure on maturation and hydrocarbon generation reactions in geological basins. Unusual earlier studies (at least in terms of petroleum geochemical pyrolysis research) used a vessel in which the pressure is entirely derived from liquid water rather than both liquid water and vapour, as in hydrous pyrolysis experimentation. Results from these experiments showed that both total organic carbon (TOC) and hydrogen index (HI) were elevated in the pyrolysed kerogen residue, suggesting that hydrocarbon generation was being retarded by the effect of water pressure. This paper presents the results of an experimental investigation into the effects of water pressure and phase on hydrocarbon generation and expulsion from the Kimmeridge Clay Formation (KCF) in the temperature range 310–350°C and in the pressure range 0–500 bar, and presents quantitative results both for the amounts of gas and bitumen generated and the composition of the generated gas. The experimental results show that the water pressure retards both bitumen and gas generation, with gas generation being retarded more severely than bitumen generation.

Thermal history modelling in the southern Faroe–Shetland Basin

Hydrocarbon generation is a temperature-dependent process and the prediction of hydrocarbon generation in basin modelling requires the crustal heat flow history to be predicted. Tectonically, the Faroe–Shetland Basin (FSB) has been subjected to a number of Cretaceous rifting events, followed in the Tertiary by Balder volcanism and, finally, by Oligo-Miocene inversion. In this study the thermal history of the FSB is derived using two independent methods (rifting–volcanic–subsidence history and maturation). The methodology is described using two wells (204/19-1 and 205/22-1) in the southern FSB. The vitrinite reflectance results predicted by the PresRo® model, which was devised for modelling maturation in overpressured basins such as the FSB, produced predicted vitrinite reflectances that matched the measured values. The method was then applied to a pseudowell within the Foinaven sub-basin, and the predicted vitrinite reflectance values for the Kimmeridge Clay Formation source at 8 km depth indicate that the source is still mature for oil generation due to the extensive retardation induced by the highly overpressured, argillaceous Mesozoic sediments.

Large submarine slides on a steep continental margin (Camamu Basin, NE Brazil)

Abstract: We describe a set of thin-skinned structures from the offshore Camamu Basin. The margin is about 40 km wide and the sea floor has a slope of up to 10%. The sedimentary cover is up to 7 km thick. Neocomian strata accumulated during continental rifting. Aptian strata consist mainly of coarse clastic sediment and evaporite. Late Cretaceous strata are thin or absent and the Tertiary succession is about 1 km thick, above a prominent Eocene unconformity. The Neocomian and Aptian sequences contain abundant thin-skinned structures, which are extensional near the continental shelf and compressional toward the toe of the slope. The structures have detached on Aptian evaporite and on the base of Neocomian shale. One giant slide is about 5 km thick and 60 km wide. It displays mirror symmetry about a vertical plane perpendicular to the margin, as occurs in physical models. The main phase of sliding occurred between the Campanian and the middle Eocene. The trigger may have been a regional phase of uplift and exhumation, for which there is independent evidence on the shelf and inland.

Retardation of Hydrocarbon Generation and Maturation by Water Pressure in Geologic Basins: An Experimental Investigation

Temperature-time–based first-order kinetic models are currently used to predict hydrocarbon generation and maturation in basin modeling. Physical chemical theory, however, indicates that water pressure should exert significant control on the extent of these hydrocarbon generation and maturation reactions. We previously heated type II Kimmeridge Clay source rock in the range of 310 to 350C at a water pressure of 500 bar to show that pressure retarded hydrocarbon generation. This study extended a previous study on hydrocarbon generation from the Kimmeridge Clay that investigated the effects of temperature in the range of 350 to 420C at water pressures as much as 500 bar and for periods of 6, 12, and 24 hr. Although hydrocarbon generation reactions at temperatures of 420C are controlled mostly by the high temperature, pressure is found to have a significant effect on the phase and the amounts of hydrocarbons generated. In addition to hydrocarbon yields, this study also includes the effect of temperature, time, and pressure on maturation. Water pressure of 390 bar or higher retards the vitrinite reflectance by an average of ca. 0.3% Ro compared with the values obtained under low pressure hydrous conditions across the temperature range investigated. Temperature, pressure, and time all control the vitrinite reflectance. Therefore, models to predict hydrocarbon generation and maturation in geological basins must include pressure in the kinetic models used to predict the extent of these reactions.

Petroleum systems and results of exploration on the Atlantic margins of the UK, Faroes & Ireland: what have we learnt?

Abstract The exploratory drilling of 200 wildcat wells along the NE Atlantic margin has yielded 30 finds with total discovered resources of c. 4.1×10 9 barrels of oil equivalent (BOE). Exploration has been highly concentrated in specific regions. Only 32 of 144 quadrants have been drilled, with only one prolific province discovered – the Faroe–Shetland Basin, where 23 finds have resources totalling c. 3.7×10 9 BOE. Along the margin, the pattern of discoveries can best be assessed in terms of petroleum systems. The Faroe–Shetland finds belong to an Upper Jurassic petroleum system. On the east flank of the Rockall Basin, the Benbecula gas and the Dooish condensate/gas discoveries have proven the existence of a petroleum system of unknown source – probably Upper Jurassic. The Corrib gas field in the Slyne Basin is evidence of a Carboniferous petroleum system. The three finds in the northern Porcupine Basin are from Upper Jurassic source rocks; in the south, the Dunquin well (44/23-1) suggests the presence of a petroleum system there, but of unknown source. This pattern of petroleum systems can be explained by considering the distribution of Jurassic source rocks related to the break-up of Pangaea and marine inundations of the resulting basins. The prolific synrift marine Upper Jurassic source rock (of the Northern North Sea) was not developed throughout the pre-Atlantic Ocean break-up basin system west of Britain and Ireland. Instead, lacustrine–fluvio-deltaic–marginal marine shales of predominantly Late Jurassic age are the main source rocks and have generated oils throughout the region. The structural position, in particular relating to the subsequent Early Cretaceous hyperextension adjacent to the continental margin, is critical in determining where this Upper Jurassic petroleum system will be most effective.