Integrated geochemical and Basin modeling analysis of the Jurassic and Cretaceous Hanifa Hydrocarbon system in Qatar

Abstract

Organic-rich mudstones of the Tuwaiq Mountain and Hanifa Formations of the Jurassic Hanifa Supersequence are the major source rocks for much of the oil in the Arabian Gulf. An integrated geochemistry and basin-modeling study on these source rocks and their oils was conducted to explain the distribution of petroleum accumulations within the Hanifa hydrocarbon systems of Qatar. Principal component analysis (PCA) of biomarker and stable carbon isotope data for oils and condensates delineates three major hydrocarbon systems in Qatar: 1) Jurassic Hanifa-sourced oils, 2) Cretaceous Shilaif-sourced oils and, 3) Silurian Qusaiba-sourced condensates (Figure 1). The biggest contribution to the PCA comes from Factor 1 where 63% of the variance in the data is primarily due to source rock facies variations. Oil-to-oil correlations indicate that the Jurassic Hanifa-sourced oils are present in both Jurassic and Cretaceous reservoirs. The Hanifa sourced-oils exhibit relative thermal maturity differences between fields and by reservoir age (Figure 2). In general, reservoired-oils (Arab C, Arab D, Araej, Izhara, and Uwainat reservoirs) from the Dukhan, Maydan Mahzam, and Bul Hanine fields have higher relative thermal maturities than reservoired-oils from Al Rayyan, Al Shaheen, and Idd E1 Shargi. For a particular field, the relative thermal maturity of the oil appears to be related to stratigraphy, with the least mature oil in the youngest reservoir, and oil maturity increasing with reservoir depth. This is best observed in the Idd E1 Shargi field where the oil is least mature in the Cretaceous Shuaiba and Jurassic Arab C reservoirs and becomes more mature in the older (deeper) Arab D and Uwainat reservoirs (Figure 2). The occurrence of relatively higher maturity oils in deeper reservoirs (Arab D, Uwainat, Izhara) suggests that the reservoirs were filled from top to bottom. Figure 4 shows that in the Eastern Salt Province fields, the Hanifa source rock maturity increased from early mature to peak mature since 50Ma. During this time, the deeper Izhara and Uwainat reservoirs of the Eastern Salt Province fields were charged with higher maturity oils. The close correspondence of the relative maturity of the oils in these fields with the maturity of the Hanifa source rock at present-day indicates that the oils were generated in the thick, rich Hanifa kitchen to the south of Qatar. The occurrence of these older reservoirs in a structurally higher position than the thermally mature Hanifa source rocks located in the adjacent structural lows suggests that hydrocarbons charged laterally into the deeper, older Uwainat and Izhara reservoirs located on the structural highs. Hanifa-sourced oils in the Jurassic Arab reservoirs of northern Qatar contain relatively low maturity oils. The presence of these oils is consistent with local generation from the pod of thin, rich Hanifa source rock (up to 20 feet thick with total organic carbon values up to 10%) that occurs north of the Qatar peninsula (Figure 4). At 50 Ma these thin-rich source rocks were immature and did not generate hydrocarbons (Figure 3). However, starting at 30 Ma and continuing to present day, this area passed through the early phase of the oil window, and generated low maturity oils that migrated into the Arab reservoirs. The good correspondence between the maturity of the oils and the present-day Hanifa source rock supports the local migration of low maturity oils as indicated by the short blue arrows on Figure 4. Cretaceous-reservoired Jurassic oils (Kharaib, Shuaiba, Nahr Umr, and Mauddud) from the Al Shaheen Field are of higher maturity (Figure 4). The Hanifa source rock in this area is immature at present-day (Figure 4). Thus, the higher maturity oils observed in Al Shaheen most likely migrated from the deeper, mature Hanifa source kitchens located to the south. These oils most likely migrated laterally through porous Arab carbonates up the flanks of the Qatar Arch from the southwest, south, and southeast and then vertically into the Cretaceous reservoirs through dissolution features in the Hith evaporite. Copyright 2005, International Petroleum Technology Conference.