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JOURNAL OF PETROLEUM GEOLOGY

vol. 35, no. 1, January 2012

available online: http:// wileyonlinelibrary.com

www.jpg.co.uk

Organic geochemistry and petrology of a Lower Jurassic 5 - 24

(Pliensbachian) petroleum source rock from Aït Moussa,

Middle Atlas, Morocco

V. F. Sachse (RWTH Aachen University), D. Leythaeuser, A. Grobe,

M. Rachidi and R. Littke

Impact of diagenesis on reservoir quality in ramp carbonates: 25 - 48

Gialo Formation (Middle Eocene), Sirt Basin, Libya

G. H. Swei (Nasser Nation University, Tripoli) and

Maurice E. Tucker (Durham University)

Carbonate geobodies: hierarchical classification 49 - 66

and database – A new workflow for 3D reservoir modelling

A. Jung and T. Aigner (University of Tuebingen)

Evaluation of hydrocarbon prospects using surface 67 - 84

geochemical data with constraints from geological and geophysical

observations: Saurashtra Basin, India

D. Mani, D. J. Patil, M. S. Kalpana and A. M. Dayal (CSIR, Hyderabad, India)

A general method for the consistent volume assessment 85 - 98

of complex hydrocarbon traps

A. Beha (DONG E&P, Denmark), J. E. Christensen and R.Young

International Events 100

Cover:

Main photo shows Eocene limestones of the El Garia Formation exposed at the El

Garia type locality in northern Tunisia. Mid-Eocene nummulitic grainstones form

important reservoir rocks for hydrocarbons in North Africa. Equivalent limestones in

the Sirt Basin (Libya), known as the Gialo Formation, are the subject of the paper by

Swei and Tucker on pp. 25-48 of this issue. Below are field photos of the El Garia

Formation (left and centre); and a photomicrograph of a nummulitic facies in the Gialo

Formation. Photos by Paola Ronchi except bottom right by Maurice Tucker.

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In this issue...

(continues page 4)

Sachse et al. (pp. 5 – 24) investigate the organic geochemistry and source rock potential

of Pliensbachian hemipelagic marls and limestones from surface exposures near Ait Moussa

in the central Middle Atlas, Morocco. The source rock interval in the outcropping succession

contains mostly Type I or II kerogen which is dominated by algal material; minor quantities

of terrigenous organic matter are also present. The source rocks were found to have

relatively advanced levels of thermal maturity (0.5-0.66 %VR) but below the level at which

peak oil generation takes place. However generation and migration of petroleum was

indicated by both microscope and organic geochemical observations. Bulk kinetic parameters

were determined for an OM-rich marl sample from the study area. 1D burial, thermal and

maturation modelling, calibrated against vitrinite reflectance, suggested that deepest burial

of the source rock succession occurred during the Eocene. The model indicated that this

period coincided with the latter of two phases of hydrocarbon generation; an earlier phase

occurred during the Late Jurassic – Early Cretaceous.

Eocene shallow-water limestones are important reservoir rocks in North Africa and

typically have grainstone-wackestone textures dominated by nummulitids and other benthic

foraminifera. In the Sirt Basin, northern Libya, such carbonates dominate the Gialo Formation

which is the focus of the paper by Swei and Tucker on pp. 25 – 48. Data came from

cores and wireline logs from five wells at the Sahl and Assumood gasfields. Six principal

carbonate facies in the Gialo Formation are identified and described, together with 14

microfacies. Deposition is interpreted to have taken place on a ramp-type platform with

km-scale foraminiferal banks and quieter-water back-bank lagoons; the ramp deepened in

places to mudstone-dominated forebank and open-marine areas. The most common cement

type observed in the Gialo Formation carbonates is sparry calcite, interpreted to have

been precipitated during shallow to moderate burial. Enhanced porosities (up to 37%)

developed mainly as a result of the dissolution of bioclastic grains and matrix during burial

at similar depths.

Jung and Aigner describe a new classification scheme for depositional carbonate geobodies

on pp. 49 – 66. Within the scheme, geobodies are assigned to six hierarchical categories,

namely: geological age; type of carbonate platform (e.g. ramp or shelf); type of facies belt;

overall geometry; type of architectural elements present; and litho-and biofacies. The

classification has been used to build up a searchable web-based database holding more

than 600 case studies derived variously from publications, outcrop analogues, and present-

day carbonate settings. The database is at the basis of a new approach to 3D reservoir

modelling which uses multiple-point statistics (MPS) to simulate facies patterns and

geometries. The modelling workflow is demonstrated with reference to a Cenomanian

carbonate shelf which crops out in SE France, a reservoir analogue for which detailed

geological data is available in the public domain. Data from the new database was used to

construct 3D training images applicable to the open-platform, slope and basinal facies belts

identified in the outcrop area. These training images were then used to build a static 3D

reservoir model.

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Publication: April 2010

www.petgeoliraq.com

The Petroleum Geology of Iraqby A.A.M. Aqrawi (Statoil), J. C. Goff (BP),A. D. Horbury (Cambridge Carbonates) and F. N. Sadooni (Qatar Univ.)

This book provides an up-to-date review of the reservoirs,source rocks and traps thatcontrol Iraq’s petroleumsystems and resources. It willbe an invaluable source ofinformation for petroleumgeologists and researchers.

Contents:1. Introduction2. History of oil exploration

in Iraq3. Tectonics & tectonic evolution4. Precambrian and Palaeozic5. Mid Permian - mid Jurassic6. Mid to Upper Jurassic7. Cretaceous8. Cenozoic9. Petroleum Systems10. The petroleum geology

of Iraq: Future ChallengesReferencesIndex

The Petroleum Geology of Iraqby A.A.M. Aqrawi, J. C. Goff, A. D. Horbury and F. N. Sadooni

ISBN 978 0 901360 36 8 Publication: April 2010440 pages, A4 format, hard coversPrice: £100 plus post /packing from the UK

Details/ordering: www.petgeoliraq.com

published by Scientific Press Ltd, PO Box 21, Beaconsfield, Bucks HP9 1NS, UK

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Adnan A. M. AqrawiAdnan A. M. AqrawiJeremy C. GoffJeremy C. GoffAndrew D. HorburyAndrew D. HorburyFadhil N. SadooniFadhil N. Sadooni

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In this issue... (cont.)

Mani et al. (pp. 67 – 84) report on the geochemical evaluation of adsorbed light hydrocarbons

(C1-C

5) in soil samples from the Saurashtra Basin, Western India. Conventional hydrocarbon

exploration activities here are made difficult by the presence of thick (2.5-3 km) Deccan Trap

basalts. The basalts cover a Mesozoic succession including Lower Cretaceous fluvial to deltaic

shales with source rock potential. Mani et al. investigated whether light hydrocarbons, originating

from these source rocks, may have migrated up through the basalts to near-surface levels. Some

150 soil samples were collected from a 1km square grid and desorbed hydrocarbons were analysed

by standard organic geochemical techniques. The samples’ iodine content was also assessed, as

increased iodine concentrations in soils may indirectly indicate a flux of hydrocarbons migrating

from depth. Results show that elevated concentrations of non-biogenic C1-C

5 hydrocarbons

were present in the soil samples. Soil gas anomalies may be associated with structural features

such as dykes and faults providing migration conduits from depth.

A particular challenge facing the oil industry concerns the measurement and evaluation of

new prospects during an exploration campaign which, as exploration proceeds, are less likely to

be simple four-way closures. Beha et al. (pp. 85 – 98) describe a computer-based method for

assessing volume uncertainty in a prospect which can be applied to the investigation of complex

traps, i.e. those in which multiple trapping elements must work favourably and simultaneously in

order to allow access to the full hydrocarbon volume potential of the prospect. As an example,

they consider the volume assessment of a four-way closure cut by two vertical faults. The faults

can be either sealing or non-sealing, and there is therefore uncertainty about the depth of leak

points and consequently the volume of hydrocarbons present in the trap. The volume uncertainty

distribution can be modelled using a Monte Carlo simulation, given various probability predictions

and the definition of uncertainty ranges for various input parameters. The simulation allows all

possible fault seal scenarios to be integrated in the prospect assessment.