Basin Evolution and Geochemical Modeling
Multi-variate analysis of Sirte basin oil data, integrated with the correlation of source kerogen, and the application of geochemical inversion procedures.
Postdoctoral research by Dr Jonathan Redfern, Dr Ralph Burwood, and Dr Michael Cope
Introduction:
Conservative estimates suggest the Eastern Sirte Basin contains reserves in excess of 23GBOR distribute
amongst numerous smaller to larger fields (figure 1). Exploration commenced in 1959 and has resulted in the discovery of several super-giant fields, such asAmal (4.25 GBOR), Augila Nafoora (2.20 GBOR), Sarir (6.50 GBOR), Gialo (3.50 GBOR) and Messla (1.50 GBOR). However, despite these giant discoveries and a number of more modest finds, exploration drilling density is still low suggesting that significant additional reserves remain to be found.
Rationale and Methods:
The Senonian marine Sirte Shale source rock provides the dominant charge in the basin (figure 2) and is the main source for the archetypal oils recognised in the Messla, Hamid and Sarir L Fields. However, the charge
for many of the oilfields is more complex and hybrid source systems are evident. Multi-variate analysis of oil data, integrated with the correlation of source kerogen to oil using carbon isotopes, has resulted in a fuller understanding of the contributory petroleum systems.
The application of geochemical inversion procedures to the biomarker and isotope data has allowed the identification of seven end-member generic oil families and additionally a number of mixed-system hybrid oils (figure 3).
Non-marine (lacustrine) source rocks can be clearly differentiated, in addition to the major marine Sirte Shale, Rachmat and Tagrifet Formations source rock systems. Pre-Upper Cretaceous Nubian (Early Cretaceous and Triassic) lacustrine sediments in the Hameimat, Sarir and Maragh troughs are identified as significant contributors, together with less volumetrically important and currently enigmatic Harash Fm. and Antelat Fm. source rocks.
The main depositional centres in the Agedabia, Hameimat, Maragh and Sarir Troughs provide the principalsource kitchens, with timing of source maturation controlled by both Mesozoic basin fill and later Paleogene subsidence.
Figure 2: Sirte basin stratigraphic column. Click to enlarge Reservoir distribution has a distinctive facies and depth relationship, reflecting regional structural control and stratigraphic variations. The Eastern Sirte Basin is characterised by both clastic and carbonate reservoirs, and many of the larger fields have multiple pay zones, from the Pre-Cambrian to Tertiary in age, including a significant input from fractured basement.
Analysis of the geographical and stratigraphic distribution of reservoirs and source rocks, the migration / charge history, together with the trapping mechanisms, provides a preliminary picture of a complex suite of petroleum systems within the Eastern Sirte Basin. Sourcing is seen not to be exclusively dependent upon the Sirte Shale candidate, thereby enhancing the diversity of play concept and eventual prospectivity.
Selected References:
Burwood, Ralph, Libya; petroleum systems of the East Sirte Basin, 1997 AAPG international conference and
exhibition; abstracts, AAPG Bulletin, 81 (8), p. 1365, 1997. Meeting: 1997 AAPG international conference and exhibition, Vienna, Austria, Sept. 7-10, 1997.
Burwood, Ralph, Petroleum system assessment for the East Sirte Basin (Cyrenaica) oil province. American Association of Petroleum Geologists 1998 Annual Meeting Expanded Abstracts - American Association of Petroleum Geologists, 1998, illus. incl. sketch map, 1998. Meeting: American Association of Petroleum Geologists 1998 annual meeting, Salt Lake City, UT, United States, May 17-20, 1998.
Burwood Ralph, Michael Cope and Jonathan Redfern (in prep.) Petroleum Systems of the Eastern Sirte Basin (Libya).