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Natural gases vary in chemical and isotope composition as a function of their formation and migration history. Compositional and isotopic variations are often caused by mixing of two or more compositionally and isotopically different gases. Isotopic properties in gases can be used to determine the mixing ratio of the two endmembers and/or calculate the composition of the endmembers from different mixing ratios. The variations of isotopic properties of gases within a continuous reservoir are generally small but can be significant between fault blocks of one reservoir or between unconnected but closely stacked reservoirs. These inter-reservoir variations can be utilized to help solve many problems occurring during gas field development and operation: Reservoir Identification: Gases within a single reservoir are very similar, but non-communicating reservoirs can often be differentiated through isotopic differences in their gases. Isotope analyses of gases could be helpful in such cases to identify the production zone in new completions. Reservoir Compartmentalhation and Fault Block Mapping: Isotopic signatures of gases in faulted reservoirs with completions in different fault blocks can be used for better identification of sealing faults and subdivision of drainage compartments. Reservoir Allocation: Isotope analyses in commingled production could be used to allocate contributions from individual sands if isotopic differences exist between the gases from the contributing reservoirs. Gas Storage: Gas storage fields provide excellent "laboratories" for demonstrating the effects of mixing gases of different compositions. Examples are given which demonstrate the use of chemical and isotopic analyses to calculate mixing ratios of storage gas and native gas. Identification of Gas Seeps: Movement of natural gas from reservoirs to the surface may change the concentration of gas components, but the isotopic composition remains mostly unaltered (isotope changes through oxidation can be recognized through special isotopic anomalies in compound patterns). Isotope analyses can, therefore, help identify the source of surface gas leaks.

Geochemical data of oil integrated with geological and engineering data have contributed to an understanding of the mechanism and relative timing of hydrocarbon emplacement in shallow oil zones (Rubble, Ostracod & Magma) of Bahrain's Awali oil field. These zones contain heavy as well as light oil. It is observed that the different oil types reside in different reservoir compartments. To efficiently develop and produce from these reservoirs, it is required to know the current distribution of the oil types and whether they result from different charging histories (gradations developed during initial or late stage migration) and or post-emplacement alteration (i.e. water washing, biodegradation, gas stripping, gravity segregation or de-asphalting). The primary objective of the study was to determine the vertical and lateral continuity of the reservoir intervals to geographically identify areas for development drilling and EOR processes. A geochemical study involving an advanced gas chromatography technique was used which included:comparison of abundance of "inter-paraffin peaks" between the different oil samples,determination of the magnitude of the compositional differences between the oil "Star Diagrams" and "Cluster Diagrams", and"Slope Factor Analysis" where the relationship between molar concentration and molecular weight (carbon number) are compared between the oils. The Geochemical evaluation provided compositional data that helped to:Characterize the difference between the oils.Verify reservoir compartment-talization.Reveal the level of biodegradation, charging fluid type andrelative charge histories for the different fluid types.Identify the geographical distribution of the different fluid types and their relation to potential migration pathways (e.g. faults). In addition to providing information concerning the lateral continuity within specific reservoir intervals and vertical continuity between intervals, the geochemical data also indicate three charging events with oils of differing thermalmaturity:Primary: Charges of extremely immature oil wereintroduced along a nearly linear path which traverses the field from SE to NW. These lines could be the major fault (N-S) and fractures providing initial migration paths.Secondary: Oils of increasing maturity (mixtures of light& heavy oil) were introduced progressively, as the basinsubsided and the oil source matured. It is possible that the light liquids condensed from gas at the sites where theypresently occur, the oils acting as a stripping liquid.Tertiary: (Emplacement of highly mature oil). Thisprocess has occurred predominantly in the Mauddud oil bearing formation, produced primarily close to the perimeter of the field. Back Ground on Geochemical Techniques to assess Reservoir compartmentalization The geochemical approach adopted in the study is based on well established proposition that oils from discrete reservoirs tend to differ from one another in composition.[1–5] These compositional differences exist for one or more of the following three reasons:Processes like biodegradation, water washing, and evaporative fractionation that affect oil composition after oil enters a reservoir.Oil composition expelled from a source rock changes as the source interval is progressively buried and the thermal maturity of the source and the generated oil increases.More than one source rock (or source facies) may contribute oil to an accumulation.