Chapter 1. General Introduction
The Sirt Basin in north central Libya is one of the youngest sedimentary basins of the African Craton, with an onshore area of approximately 375,000 Km2 and an estimated sedimentary volume of 1.3 million km3 (Abadi et al., 2008). It is considered to be one of the richest and prolific oil-bearing basins in the world, containing more than 100 oil and gas fields, including several giants. It is considered to be the largest petroleum province so far discovered in Libya (Hallett, 2002).
More than 1600 wildcat wells were drilled in the Sirt Basin, resulting in 250 discoveries with recoverable reserves of 45 billion barrels (bbl.) of oil and 33 trillion cubic feet (tcf) of gas (Hallett, 2002). Nearly 80% of the total recoverable oil and gas were discovered in shallow plays before 1970 (Hallett, 2002). After that time the exploration activity was retarded due to lack of technologies such as advanced seismic acquisition and processing, advanced geochemical studies, computer related geoscience technology, limited understanding and application of petroleum system analysis and ineffective use of sequences stratigraphic concepts (Hallett, 2002). Therefore, achievement the aims and objectives of this geochemical study may assist exploration for oil and gas in the study area.
1.1 Background and Research Objectives
The distribution of petroleum (oil and gas) in any sedimentary basin is controlled by combination of a number of processes and elements that may have changed through the geological history of the basins. The essential elements and processes that make up a petroleum system include a petroleum source rock, thermal maturation, a migration pathway, reservoir rock, trap and seal (Magoon and Dow, 1994).
Petroleum Geochemistry continues to play a critical role to find the remaining resource that is becoming more difficult to locate, discover and produce, and it is clear that geochemical analysis is increasingly becoming a vital tool for minimizing exploration risk (Peters, 2005b).
Despite the considerable amount of basic geochemical analyses that has been done in the Sirt Basin, the geochemical knowledge of this basin is still incomplete and not well
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understood, because this wealth of data has not yet been incorporated with information from modern geochemical analytical techniques, such as biomarker data from Gas Chromatography–Mass Spectrometry MS), specific carbon isotope analysis (GC-IRMS) data and basin modelling analyses using these data.
These techniques, which are now becoming widely applied in geochemical investigations, have the possibility of resolving questions regarding the timing of oil generation, thermal history and subsequently, secondary migration distance and pathways of the oil from source rock to trap within the Sirt Basin. Due to the exploration and production complexities that encountered by most of the operations oil companies working for exploring on the oil and gas in the Sirt Basin, particularly in the central and southwest region, therefore it becomes necessary to perform this geochemical research study that may help to solve such kinds of problems.
Secondary migration distance and pathway information is not achievable using seismic surveys but (Larter and Aplin, 1995) illustrated that the organic geochemistry can be used to resolve petroleum migration history, to determine the volume of petroleum migration and volumes of water interacted with it and to distinguish between oils which have migrated throughout the pore system in fine grained rocks and those have migrated via fractures and faults (Larter et al., 1996).
In general, organic compounds that have different molecular weights, polarities and stereochemistries should behave differently during the various adsorptive processes which may take place during the migration of petroleum in the subsurface. Seifert and Moldowan (1978) used biomarker alkanes to describe regional petroleum migration pathway but (Yamamoto et al., 1991) showed that parameters derived from biomarker alkanes are affected by other factors such as variation in source organic matter input, depositional environments and thermal maturity. Peters and Moldowan (1993) also noted limitations of biomarker alkanes for petroleum migration assessment in the subsurface.
Subsequently, several geochemical studies were carried out in both field and laboratory have shown potential indicators of petroleum migration distance in more polar compound in petroleum; such as aromatic nitrogen compounds including carbazoles
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and benzocarbazoles (Yamamoto et al., 1991; Li et al., 1992; Li and Larter, 1993; Li et al., 1994; Larter and Aplin, 1995; Li et al., 1995b; Larter et al., 1996; Li et al., 1997;
Taylor et al., 1998).
Aims and Objectives
This study concentrates on molecular marker signatures for identifying those phenomena mentioned above in order to better understand these processes and thus the whole petroleum system of the Sirt Basin.
A number of objectives were planned in order to achieve this broad aim.
To determine the number of genetically distinct oil families in the basin,
Utilize geochemical characteristics of the oil families to infer their source facies,
Determine thermal maturity level and degree of preservation,
Determine the most likely source unit in each part of the study area by comparing the distribution of oil families and their inferred source facies with regional stratigraphic and available source rock data,
Attempt to correlate each oil family to the specific source stratigraphic unit,
Attempt to predict oil properties at different maturity levels,
Estimate migration directions by comparing oil family distributions with the location of known oil kitchens source rock within the study area,
Determine the timing of hydrocarbon generation within each source rock.
In order to realise these objectives, routine and advanced geochemical techniques were carried out including gas chromatography-mass spectrometry (GC-MS) for biomarker analyses, compound specific carbon isotope analyses (GC-IRMS) and basin modelling analysis. Achievement of these aims and objectives will reduce the risks that associated with the exploration activity within the study area in the following ways.
1. Assessment of the maturity-levels of effective source rocks and improve the understanding of the vertical and lateral distributions of the major oil kitchens.
2. Establishing the source rocks and kerogen types responsible for generating and expelling these petroleum fluids (i.e. likelihood of oil or gas).
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3. Inputs to determine the filling history and timing of charging of the reservoirs in the study area (basin modelling).
4. Estimation of the volumes of oil and gas generated and expelled from effective source rocks.
5. Determine the relationship between effective source rock(s) and oil(s) as well as oil family with each other (i.e. oil to source and oil-to-oil correlations).
6. To suggest the migration pathways and filling directions of the fields across the study area.