ALBERT

Description: The unpredicted discovery of downdip oil in Permian–Carboniferous Unayzah sandstone reservoirs along the southern flank of the predominantly gas condensate Ghazal field in Saudi Arabia highlighted the need for a thorough assessment of controls on hydrocarbon properties, with the aim to improve prediction and reduce uncertainty. Primary controls on hydrocarbon property variations (condensate-to-gas ratio) here appear to be compartmentalization, multiple charging, presumably from a Lower Silurian Qusaiba kitchen to the south and east within the late-oil to wet-gas generation window, and dysmigration to the north and west. Gas on production represents mostly unmixed thermogenic accumulations of variable maturities, with no sign of biodegradation or thermal cracking. Light oil (1.06–1.21% calculated vitrinite reflectance [Rc]) in the southern part of the field represent late-mature accumulations that apparently escaped flushing by subsequent gas condensate charges generated near the wet-gas generation window (1.41–1.55% Rc) and currently occupying the field to the north. Constant maturity variations for both gasoline range and heavier components in each sample testify to a lack of mixing between the hydrocarbon phases. Petroleum inclusions similarly comprise either light oil or gas condensate that coexist with significantly overlapping homogenization temperatures, suggesting considerable overlap in their trapping history (135–35 Ma). Less mature petroleum has not been identified in the field, possibly because of a lack of trap closure or migration focus prior to the inferred Late Jurassic filling or because of displacement to updip structures, thereby high grading the probability of undiscovered oil west and northwest of Ghazal. If thermal generation instead of retrograde condensation or phase separation controls this petroleum system, then the potential for deeper oil accumulations is limited. Khaled R. Arouri is an exploration petroleum geochemist and geologist at the EXPEC-ARC. He has 20 years of diverse research, consultancy, and teaching experience in the Middle East and Australia, focusing on using organic geochemistry as a tool in petroleum exploration and field development. He received his Ph.D. (1996) from the University of Adelaide, Australian School of Petroleum, and is a member of the AAPG and European Association of Organic Geochemists (EAOG). Pierre Van Laer joined Saudi Aramco in 2002, working in exploration as basin modeler and stratigraphic modeler, and being responsible for resource assessment. Previously, he worked as senior research scientist for Shell (1998) and Eni (1991), where he developed basin modeling softwares and provided service works worldwide and technology transfer to the operating units. After receiving his Ph.D. in carbonate sedimentology (University of Liege, 1989), he worked at Institut Geographique National (IGN) Brussels in remote sensing and GIS. Mark H. Prudden is a senior development geologist in the reservoir characterization department, which is responsible for the fast-track development of the recently discovered gas fields. He has more than 25 years of international experience, working previously with Production Geoscience Ltd. (PGL), Schlumberger, and Statoil, and has been in Saudi Aramco since 2001. Mark has an honors degree in geology (1980) from the University of Dundee, Scotland. Peter D. Jenden has worked at Saudi Aramco for 8 years providing analytical and interpretive support in petroleum geochemistry. Prior to that, he spent 13 years with Chevron Oil Field Research Company and Chevron Canada Resources. His current focus is on applications of the stable isotope ratios of H, C, N, and O and on the origin of natural gas. He is a member of the AAPG and the European Association of Organic Geochemists (EAOG) and has a Ph.D. in geochemistry from the University of California, Los Angeles. William J. Carrigan is a geologist and petroleum geochemist who recently took early retirement from the EXPEC-ARC of Saudi Aramco, which he joined in 1990 after working previously with the Geological Survey of Canada and with several independent consulting companies. He has a B.Sc. in geology from Queens University and a Ph.D. in geology from the University of Ottawa, Canada. Adnan A. Al-Hajji has worked for Saudi Aramco since 1976 and is currently a science consultant at the research and development center, focusing mainly on mass spectrometry and petroleum geochemistry. He holds a B.Sc. degree in industrial chemistry and an M.Sc. degree in chemistry from King Fahd University of Petroleum and Minerals (KFUPM), Saudi Arabia.

Description: Twenty-seven kerogen samples from the Guaymas Basin were examined to determine the effects of dolerite sill intrusions on recently deposited organic matter. ESR spin density and line width were observed to pass through maxima during the course of alteration. ESR g value, however, showed no correlation with maturity. Rather surprisingly, d13C decreased by 1 to 1.5 per mil in the vicinity of the sills at Sites 477 and 481. In addition, atomic N/C decreased only slightly with proximity to a smaller sill at Site 478. Differences in maturation behavior between Sites 477 and 481 and Site 478 are attributed to dissimilarities in thermal stress and to the chemical and isotopic heterogeneity of Guaymas Basin protokerogen.

Description: The effects of intrusive thermal stress have been studied on a number of Pleistocene sediment samples obtained from Leg 64 of the DSDP-IPOD program in the Gulf of California. Samples were selected from Sites 477, 478 and 481 where the organic matter was subjected to thermal stress from sill intrusions. For comparison purposes, samples from Sites 474 and 479 were selected as representative of unaltered material. The GC and GC-MS data show that lipids of the thermally unaltered samples were derived from microbial and terrestrial higher-plant detritus. Samples from sill proximities were found to contain thermally-derived distillates and those adjacent to sills contained essentially no lipids. Curie point pyrolysis combined with GC and GC-MS was used to show that kerogens from the unaltered samples reflected their predominantly autochthonous microbial origin. Pyrograms of the altered kerogens were much less complex than the unaltered samples, reflecting the thermal effects. The kerogens adjacent to the sills produce little or no pyrolysis products since these intrusions into unconsolidated, wet sediments resulted in in situ pyrolysis of the organic matter. Examination of the kerogens by ESR showed that spin density and line width pass through a maximum during the course of alteration but ESR g-values show no correlation with maturity. Stable carbon isotope (d13C) values of kerogens decrease by 1-1.5 per mil near the sills at Sites 477 and 481 and the atomic N/C decreases slightly with proximity to a smaller sill at Site 478. Differences in maturation behavior between Site 477 and 481 and Site 478 are attributed to dissimilarities in thermal stress and to chemical and isotopic heterogeneity of Guaymas Basin protokerogen.

Description: Low salinities and strong opposing fluctuations between chlorinity and interstitial water d18O and dD characterize the region of gas-hydrate occurrence at Site 533. In addition to the hydrate recovered from 243 m sub-bottom, thin, diffuse layers of hydrate composing about 10% of the total pore space may have been cored around 290 m. Decomposition during recovery could account for the high oxygen isotope value (+2.80 per mil) and low chlorinity (17.21 g/kg) observed at this depth. High alkalinity and dissolved ammonia (71 meq/l and 33 mM, respectively) indicate substantial microbial activity. Chlorinity gradually increases with depth in the 1600-m-thick sediment column at Site 534, perhaps due to deepseated alteration-hydration reactions or to the presence of nearby evaporites. d18O and dD decrease with depth and are well correlated, suggesting that a common mechanism is fractionating both.