Abstract Reservoir geochemistry evolved during the late 1980s and early 1990s during one of the many downturns in exploration activity. A landmark paper in 1989 by William England of BP was one of the primary catalysts for the emergence of geochemistry as a tool for reservoir characterization. Among the many concepts appearing in that paper was the idea that compartments within a reservoir could be distinguished through the use of geochemical maturity parameters since oil in the different compartments had been generated at different levels of source rock maturity. At the same time, the geochemists at Chevron were utilizing high resolution gas chromatography of crude oils to demonstrate whether or not oils in different fault blocks or compartments were in communication. The increase in interest in applying geochemistry to reservoir characterization was manifested by numerous papers using many different techniques and concepts applied to a variety of reservoir problems. Many of these ideas quickly fell by the wayside but those that had real application were well received in the industry and are still in widespread use today. Not all of these techniques are necessarily connected with communication between fault blocks but may cover topics such as wax accumulation; asphaltene precipitation; biodegradation; effects of water washing; and numerous other problems. It is also important to remember that geochemistry can be applied to characterization of gas reservoirs as well as oil reservoirs. In the same way as oil reservoirs, continuity and compartmentalization in gas reservoirs are two areas where geochemistry can play a key role. With gas samples, this is typically done through a combination of carbon and hydrogen stable isotopes. The development of the combined gas chromatograph–isotope ratio mass spectrometer now permits one to determine the isotopic composition of individual compounds, and as a result, it is a relatively facile process to determine the isotopic composition of the individual compounds in a natural gas sample. The purpose of this paper is to review the developments in reservoir geochemistry over the past two decades and to highlight this with examples of where geochemistry has been used successfully as one tool to address reservoir problems.

Abstract Over the past 10 years, investigations into the characteristics of the high molecular weight hydrocarbon (HMWHC) fraction in crude oils and, to a lesser extent, source rock extracts have continued to reveal novel information concerning the distribution of hydrocarbons >C 40 . The major impetus for this work has come from the fact that HMWHCs can cause significant production problems in certain geographical regions and particularly deepwater frontier areas. Since these HMWHCs appear to be ubiquitous in crude oils, the primary questions that need to be addressed are: what are these compounds, where do they come from, and how do they affect physical properties of oils? Here, we review our work over the past decade and discuss the significance of these results and their potential application to reservoir and production problems involving paraffins and asphaltenes. It was commonly believed for many years that only oils derived from source rocks containing higher plant source material would have a high paraffin content. However, it is now abundantly clear that oils derived from lacustrine and marine source rocks also contain relatively high concentrations of higher molecular weight hydrocarbons. In addition to developing methods for the qualitative and quantitative separation of HMWHCs from asphaltenes, progress has been made in identifying individual components of the high molecular weight fraction. This fraction is not a simple mixture of n -alkanes but a complex mixture of seven or eight homologous hydrocarbon series, each with significantly different physical properties. A knowledge of these structures is important in predicting crude oil properties such as cloud point and pour point. Series identified to date include alkylcyclopentanes, alkylcyclohexanes, alkylbenzenes and various branched hydrocarbons. In summary, since the 1970s most of the geochemical research emphasis has been placed on compounds below C 40 . Whilst compounds above C 40 may not have the same degree of structural specificity as the traditional biomarkers, the amount of information available from these compounds could be extremely beneficial in the long term, particularly for reservoir characterization and production purposes and other problems involving high molecular weight hydrocarbons.

Abstract: Detailed analysis of a sedimentary sequence of the Lorca basin (Upper Miocene, southeast Spain), employing stratigraphy, sedimentary petrology, paleontology and organic geochemistry, permitted the establishment of sediment-biomarker relationships of deposits formed under highly variable conditions. The conditions of sedimentation in this sequence range from open marine to strongly hypersaline (both marine and non-marine), as well as a number of marked variations in circulation, water-body chemistry and salinity. Most of the sectíon formed under marine conditions, but the upper part of the sequence is the product of increasingly non-marine waters, suggesting that the basin sporadically may have become a non-marine, hypersaline lake. Sedimentologically, many details of water circulation and the increasing amount of non-marine input are not particularly apparent, but the distribution of n -alkanes and isoprenoids permit these changes to be evaluated in a more convincing fashion.