Henry I. Halpern

Development and Applications of Light-Hydrocarbon-Based Star Diagrams

Two star diagrams (multivariate plots in polar coordinates) have been developed to assess subtle chemical differences among oils. The diagrams are based on analysis of C7 hydrocarbons of crude oils separated by gas chromatography. The first of the C7 diagrams is used for distinguishing variations in oils caused by transformation, e.g., water washing, biodegradation, and evaporation. Eight ratios are plotted on this diagram in order of decreasing sensitivity to biodegradation. The ratios are designated Tr1 through Tr8, and the diagram is referred to as the C7 oil transformation star diagram (C7OTSD). A second C7-based diagram consists of five ratios composed of compounds that are very resistant to the effects of transformation, and thus are useful for correlation. These ratios, designated C1 through C5, are plotted on the C7 oil correlation star diagram (C7OCSD). This diagram can be used to correlate both transformed and primary oils based on source-related chemical differences, a task not easily accomplished through most conventional geochemical techniques. For example, condensates often contain no biomarkers due to their advanced maturity and therefore cannot be correlated with lower maturity oils. However, both condensates and oils usually contain light ends allowing correlation by C7-based star diagrams. The division of star diagrams into two types allows one to assess the relative degrees of transformation among a set of oils that have been found to correlate based on the C7OCSD. These microscale correlations detect small chemical differences between oils; and, therefore, allow even slight variations among oils within the same reservoir to be recognized. The technique has many applications that concern both exploration and production. Production-related uses include reservoir compartmentalization studies, investigating reservoir extent and connectivity, determining the source of casing leakage, and determining the extent of commingling of oils from different reservoirs in multicompletion wells (or leaky single-completion wells). Exploration problems that can be addressed are mechanisms and models of field-fill history and evaluation of oil quality in untested pay zones. Most of these problems can be addressed through simple, cost-effective analyses on produced or tested crude oils. For example, a large advantage in production applications is the ability to assess reservoir connectivity without halting production from any well. An additional use of the technique is for investigating contaminants or incompletely refined crudes in refinery streams. In this paper, I present several case studies that have already been done at Saudi Aramco. These studies include three examples of behind-casing leakage, two examples of oil differentiation, and a single example each of correlation of condensates and refinery contaminant tracing.

Organic Geochemistry and Oil-Source Correlations, Paleozoic of Ohio

Routine analytical methods and new kerogen-kerogen pyrolyzate techniques used for detailed petroleum geochemical studies permit the correlation of oils to their respective source rocks in Ohio. In the Paleozoic section, four good to excellent source units were identified: the Mississippian Sunbury Shale, the Devonian Ohio Shale and Olentangy Shale, and the shales of the Ordovician Point Pleasant Formation. These marine shales are dominantly oil-prone, with maturation levels that span the immature to peak oil-generation range. No source rocks examined exceeded an Ro of 1.0-1.1% (or equivalent). The reservoired oils in Ohio are mostly supermature, based on normal alkanes, pristane/n-C17 to phytane/n-C18 ratios, low asphaltene and sulfur contents, and high paraffin indices. Three basic oil groups were identified using oil characterization procedures: group 1 consists of Cambrian, Ordovician, and some Silurian oils; group 2 consists of some Silurian, Devonian, one Mississippian, and some Pennsylvanian oils; and group 3 consists of oil from the Mississippian Berea sandstone. Oil-source rock correlation techniques permitted the assignment of probable source rocks to each oil group. Group 1 oil was probably derived from the Point Pleasant shales, group 2 oil was derived from the Ohio Shale and Olentangy Shale, and group 3 oil came from the Sunbury Shale. Because the supermature oils in Ohio occur in reservoirs with maturities generally under 0.8% Ro, the oils would have had to migrate from deep within the Appalachian basin. Migration could have followed fracture zones, unconformities, or extensive and permeable units such as the Silurian Clinton sands.

Geochemical variations among eastern Saudi Arabian paleozoic condensates related to different source kitchen areas

Abstract Large reserves of gas and condensate are found in Paleozoic reservoirs (Khuff, Unayzah and Jauf) in the Ghawar area of eastern Saudi Arabia. The source rock for these hydrocarbons is the basal organic-rich “hot shale” of the Qusaiba member of the Silurian Qalibah Formation. This is the same prolific source rock that generated the sweet light crude oil and condensate found in the Unayzah reservoirs in central Saudi Arabia. Thermal maturity modeling of the source rock identifies two kitchen areas: to the east and west of the Ghawar structure. The west kitchen was well into the oil window by 210 Ma, while the east kitchen was just entering the oil window. Large areas of both kitchens were in the wet gas window by 125 Ma and by 90 Ma most of the area north of Ghawar was generating dry gas. In eastern Saudi Arabia, Paleozoic oil and condensate are found in early-formed structures (Carboniferous and early Triassic growth), whereas late Cretaceous and Tertiary structures contain only dry gas. The geochemistry of the oils and condensates indicates two groups, each generated from a different hydrocarbon kitchen, suggesting that the two kitchens differ slightly in organic facies. Within each group, variations among hydrocarbons are related to the thermal maturity of the source rock during generation. Within the Khuff reservoir, gas souring is caused by thermochemical sulfate reduction.

Petroleum geochemistry of the Midyan and Jaizan basins of the Red Sea, Saudi Arabia

Abstract During the 1960s, petroleum exploration activities in the offshore Red Sea areas of Saudi Arabia tested gas and condensate reservoired in the Miocene sands immediately below the Mansiyah evaporites in the offshore Midyan basin. Recent onshore exploration activity in the Red Sea has resulted in the discovery of accumulations of oil, gas and condensate in the Lower Miocene Maqna Group in the Midyan and Jaizan basins. As a result of this exploration success, an effort to understand the origin of these hydrocarbons was initiated. The two basins were assessed geochemically by addressing: (1) the potential source rocks; (2) the extent of the hydrocarbon kitchens; and (3) characterization of the hydrocarbons. The potential source rocks for the reservoired hydrocarbons are: (1) the organic-rich, oil-prone shales of the predominantly evaporitic Mansiyah Formation; (2) the variable quality shales and carbonates of the Magna Group; and (3) the moderately organic-rich shales of the Burqan and Tayran Groups. The reservoired hydrocarbons were characterized by carbon isotopes, gas chromatography-mass spectrometry and gas chromatography and compared with the potential source rocks. The results showed an acceptable match to the Maqna and Burqan organic-rich units. Detailed burial history/thermal modelling projects were undertaken to assess the hydrocarbon kitchens of both basins. Results for the Midyan basin indicated that over large areas Tayran and Burqan sediments are oil to gas mature and may be sources for the gas and/or condensate accumulations, whereas the limited area of mature Maqna sediments may be responsible for sourcing the black oil accumulations. In the Jaizan basin, the Maqna and Burqan sediments range from high oil maturity to thermally spent due to high geothermal conditions and excessive burial. The burial of the source rocks increases fairly rapidly from east to west in the Jaizan basin.

Geochemical Study of Tar in the Uthmaniyah Reservoir

Tar is believed to be the main factor impeding production in certain regions of the Uthmaniyah area, Ghawar field. The effect of extraction with several solvents on the permeability and porosity of core plugs from tar zones in the Arab-D Formation was determined in order to understand to what extent tars contribute to obstructing the rock pores. Thin section examinations of the extracted rock were conducted to discern where the tar was distributed microscopically and how that distribution corresponded with the permeability and porosity data. The data show that in general, while organic matter continued to be removed by increasingly polar solvents, the effective permeability, which is controlled by the macropore system, showed little improvement. While the major pore network and macroporosity can generally be improved in the initial extraction, the marginally accessible porosity is still largely occluded by tar. Elemental and pyrolytic analyses of core samples before and after extraction indicate that the tar is neither itself homogeneous, nor uniformly distributed through an individual well, or from well to well. Some components of the tars are not soluble to any of the organic solvent systems utilized, and evidence that some of the tar may result from thermochemical sulfate reduction (TSR) is presented.

Saudi Aramco's Real-Time Tar Mat and Oil Reservoir Characterization via GS-ROX: Application Examples

Saudi Aramco has developed fully commercial methods to quantitatively assess reservoir oil quality, productivity, water saturation, and tar identification and quantification from residual hydrocarbon staining on drill cuttings. The patented methods are known collectively as Pyrolytic Oil-Productivity Index (POPI) Technology. Unlike results from wellsite logging tools, which provide an indirect assessment of the rock properties, pore system, and fluid properties in a reservoir, pyrolytic methods provide a direct assessment of the residual hydrocarbons present in rock samples and can be used to assess connectivity with the active fluid system in the reservoir.

Ohio Paleozoic Source-Reservoir Combinations: Source Rock Quality and Source-Oil Correlation Studies: ABSTRACT

End_Page 1916------------------------------Although relatively simple structurally, the Interior Lowland area underlying Ohio and adjacent states constitutes a rich and varied hydrocarbon habitat. Structural style included influences of three subsidence episodes, broadly encompassing the Appalachian orogeny to the east and the Michigan and Illinois basins to the northwest and southwest, respectively. A sedimentary sequence covering the whole Paleozoic succession is variously present and becomes generally younger toward the southeast. Hydrocarbons are produced from numerous reservoir intervals within this Paleozoic section. Prominent among these are the Cambrian-Ordovician Knox Group, Ordovician Trenton Limestone, Silurian Medina Group, Devonian Oriskany and Vanango Sandstones, Mississippian Berea Sandstone, and Pennsylvanian c al measure sands. A variety of petroleum types, implying an equal variation in source rock characteristics, has been recognized. Reservoirs have been charged variously from finely textured organic-rich source beds cosedimented within the same succession. Whether the simplistic case of source charging of syndepositional or directly adjacent reservoir beds is normal or whether more complex long distance lateral and/or vertical emplacement processes are involved has yet to be subject to definitive study. Some of the more prominent source candidate rocks include the Conasauga Shale (Cambrian), Reedsville or Utica Shale (Ordovician), Ohio Shale (Devonian), and Bedford or Sunbury Shale (Mississippian), in addition to various Pennsylvanian intervals. Using kerogen pyrolysis-carbon isotopic source-oil correlation technology, it is possible to match petroleums with their precursor sources. End_of_Article - Last_Page 1917------------