WEST OF SCOTLAND
Thermal and tectonic development and hydrocarbon generation history assessed using AFTA^® and vitrinite reflectance

NON-EXCLUSIVE PROJECT

1. INTRODUCTION

Geotrack initiated the West of Scotland project in May 1995 to acquire the thermal history data needed to recognise and characterise the thermal episodes that have affected the region, and assess the development of maturity, hydrocarbon generation and tectonic evolution. The data produced by the project are critical to understanding and predicting oil and gas generation in the region.

The project is based on 124 AFTA and 128 vitrinite reflectance samples from 11 wells and 39 offshore shallow boreholes from the Malin, Hebrides, Minch, West Orkney and Outer Hebrides regions and from outcrop locations in the Hebridean Isles, Skye, Mull and the Scottish mainland. The project has been funded by several groups of oil companies and includes data and analyses of samples from four non-released wells provided by supporting companies. Relevant data from recently completed studies in adjacent areas in the North Channel and West of Shetlands are integrated with the new data to provide continuous thermal history data coverage along the Western margin of the UK continental shelf.

The final report presents all the AFTA and VR data and a rigorous interpretation of thermal history and its effects on maturity development and hydrocarbon generation, providing the maximum amount of hard data on thermal history of the region that modern methods allow. The report can be purchased on a non-exclusive basis. This document provides background to the project, details of the database and prices.

2. BACKGROUND TO THE PROJECT

The onshore and offshore basins north and west of Scotland represent over 100,000km² of the UK continental shelf that is virtually unexplored by drilling. The region contains a complex of basins containing Permian, Triassic and Jurassic sequences. Onshore, the Jurassic contains thick source rock sections and oil stained sandstones. Source rocks also occur in the Upper Carboniferous section at outcrop, which can be inferred to underlie large parts of the region.

Hydrocarbon prospects depend critically on the thermal history and its control on maturity development and hydrocarbon generation through the history of these basins. Prospectivity also depends on the tectonic evolution, particularly the history of uplift and erosion with its consequent effects on source rock maturation and reservoir distribution.

AFTA studies of adjacent regions in the Irish Sea, North Channel and West of Shetlands have revealed complex thermal histories. Several episodes of heating and cooling have affected the Upper Paleozoic and Mesozoic sections. In the Irish Sea for example, known hydrocarbon accumulations are restricted to those areas where paleothermal episodes late in the history dominate the maturation of Carboniferous source rocks. The West of Scotland region has possible analogies with the Irish Sea, both in the potential for Carboniferous source rocks and the in the possibility of dominant paleothermal episodes late in the history linked with igneous activity. Potential analogues also exist in the West of Shetlands region, where paleothermal episodes affecting the Jurassic source rocks show complex but consistent patterns.

This project sets out to link the thermal history data from the wells in the southern part of the West of Shetlands area through the Outer Hebrides Shelf and Western Isles into Northern Ireland and the North Channel, hence filling a large gap in our knowledge of the thermal and tectonic evolution of the UK western continental shelf.

3. STUDY OBJECTIVES

The primary study objectives are to:

• Apply AFTA and VR data to recognise the dominant paleothermal episodes and identify their geological timing and geographical distribution.
• Reconstruct paleotemperature profiles and determine paleo- geothermal gradients from vertical sequences of AFTA and VR samples in wells, and evaluate to help establish the origins and mechanisms of the heating.
• Determine maximum and peak paleotemperatures and their geological timing. Establish a regional thermal history framework within which the history of hydrocarbon generation can be understood.
• Assess levels of maturity in Jurassic and Carboniferous source rocks and determine when this maturity was developed.
• Assess the effects of the Tertiary igneous activity, including localised heating; widespread effects due to convection and fluid flow, possible associated high basal heat flow.
• Identify the timing of the significant phases of uplift and erosion; estimate the amount of eroded section; assess the significance
• Produce a regional synthesis of thermal history, maturity development and hydrocarbon history.

4. AFTA AND VR DATA AND SAMPLE DETAILS

AFTA data have been acquired and processed to the highest industry standard by Geotrack International. Apatite yields from the samples analysed are generally good to excellent resulting in high data quality. Data interpretation has been carried out using Geotrack's recently introduced multi compositional apatite annealing model which makes rigorous allowance for the controls of apatite composition on annealing rates. This includes microprobe analysis of all apatite grains and compilation of chlorine content data for each sample. In wells 134/5-1 and IR 12/13-1 where pre-existing AFTA data are being used, the data have been re-evaluated using the multi compositional model and the most up-to-date interpretation system.

The vitrinite reflectance data has been acquired to a high standard by Prof. Alan Cook, widely regarded as one of the most experienced analysts currently practising. The data have been produced using the exacting Romax method, in common with other Geotrack studies. The VR data are presented with full organic maceral descriptions and data histograms.

The number of samples taken for AFTA and VR for each stratigraphic unit are shown below in brackets. Contact us for the location of wells, shallow boreholes and outcrop samples.

EXPLORATION WELLS:

132/15-1 IR 12/13-1

AFTA - Tertiary (5), Archaean (1) AFTA - Cretaceous (3), Triassic (3)

VR - Tertiary (14) VR - Tertiary (2), Cretaceous (3), Jurassic (1), Triassic (5)

134/5-1 IR 13/3-1

AFTA - Jurassic (3), Triassic (5) AFTA - Carboniferous (4), Igneous rock (1)

VR - Jurassic (26), Triassic (3) VR - Carboniferous (16)

156/17-1 202/19-1

AFTA - Triassic (4), Torridonian (1) AFTA - Permo-Triassic (9)

VR - Jurassic (5)

164/25-1 Port More-1

AFTA - Permo-Triassic (3), Tertiary (10) AFTA - Triassic (4)

VR - Tertiary (16), Cretaceous (7) VR - Jurassic (4)

164/25-2 Magilligan-1

AFTA - Tertiary (5), Archaean (1) AFTA - Triassic (2), Carboniferous (1)

VR - Tertiary (10) VR - Jurassic (1), Triassic (1), Carboniferous (5)

SHALLOW BOREHOLE AND SEA BED CORE SAMPLES:

AFTA: VR:

BH71/09 Devonian BH72/33 Jurassic

BH71/10 Jurassic BH76/55 Jurassic

BH71/11 Permo-Triassic BH78/02 Jurassic

BH71/12 Jurassic BH78/04 Jurassic

BH71/13 Torridonian BH85/08 Jurassic

BH71/14 Permo-Triassic

BH72/32 Permo-Triassic

BH78/02 Jurassic

BH78/05 Triassic

BH81/12 Permo-Triassic

BH81/13 Triassic?

BH88/04 Triassic

BH88/08 Triassic

BH90/08 Permo-Triassic

BH90/09 Permo-Triassic

BH90/12A Permo-Triassic

BH90/14 Lewisian

BH90/16 Triassic

94 Permo-Triassic

642 Jurassic

666 Tertiary?

OUTCROP SAMPLES

AFTA:

Jurassic (9)

Lewisian (6) Cretaceous (2)

Torridonian (4) Tertiary (4)

Dalradian and Moine (6)

Devonian (2)

VR:

Carboniferous (2) Jurassic (7)

Permo-Triassic (3) Carboniferous (1)

For further details please contact us