Steve Larter

Relationship between reservoir diagenetic evolution and petroleum emplacement in the Ula Field, North Sea

Abstract The diagenetic and petroleum filling histories of the Ula Field have been studied by analysing aqueous and petroleum inclusions occurring in authigenic cements. This study shows that diagenesis continues actively after the arrival of petroleum in the sandstones, although the reaction rates and petroleum saturation remain obscure. Microthermometric measurements of fluid inclusions in authigenic quartz suggest an onset of extensive quartz cementation at temperatures around 110°C (i.e. ≈ 2.5 km) and that this cementation has continued to the present day. Synchronously with quartz cementation, authigenic albite was formed as inter- and intragranular cements. Large amounts of petroleum inclusions locally occur in the albite and quartz cements. These inclusions were trapped in the cements as the main charge of petroleum arrived at the structure. Diffusion, through thin water layers between the petroleum and the mineral surface, would probably have dominated mass transfer of solutes for the precipitation of authigenic quartz and albite. It is evident from analysis of the petroleum inclusions in early formed K-feldspar overgrowths that some secondary migrated petroleum probably arrived in the reservoir at a shallow burial depth, before the rapid subsidence in the last 25 my. Compared with petroleum in the free porosity of the Ula Formation, the petroleum in the K-feldspar inclusions is evidently sourced from a different source facies and most maturity parameters testify to the latter being less mature. Later trapped petroleum inclusions, in quartz and albite, have characteristics found both in K-feldspar and in the Ula Formation DST oil and, thus, is likely to reflect the progressive change in the Ula Field petroleum charge which occurred during the time period of quartz diagenesis.

Degradation and maturity controls on oil field petroleum column heterogeneity in the Gullfaks field, Norwegian North Sea

Abstract A geochemical mapping of the Gullfaks reservoir petroleum column based on 226 reservoir core extracts, and 23 drill stem test (DST) and repeat formation test (RFT) oil samples indicates the presence of statistically testable lateral compositional heterogeneities in the petroleum column. These different petroleum populations can be related to lateral filling of the field from two source basins and a more pervasive, subsequent episode in-reservoir biodegradation. Petroleum maturity gradients within the field were detected using conventional biomarker GC-MS methods on saturated hydrocarbon fractions. The degree of biodegradation was monitored by quantitative GC of n -alkanes in whole oil samples, using a squalane internal standard. The observed compositional differences in the Gullfaks petroleum column are consistent with diffusion being the primary mixing process, thermal convection being eliminated on both compositional and modeling grounds. Compositional differences between core extracts and DST samples from the same intervals suggest that fractionation of petroleum might occur during testing, and that interpretation of reservoir petroleum columns from mixed sample sets should be treated cautiously.

Some pragmatic perspectives in source rock geochemistry

Abstract Conceptual advances in petroleum source rock geochemistry made during the last decade are reviewed and some problem areas in analytical geochemistry are outlined. Areas of discussion include views of source rocks based on assessment of hydrocarbon expulsion, kinetic models of kerogen maturation/vitrinite reflectance evolution and a discussion of the data dilemma facing modern source rock geochemistry. The increased application of petroleum geochemistry to the quantitative solution of geological problems is stressed.

Determination of Petroleum Accumulation Histories: Examples from the Ula Field, Central Graben, Norwegian North Sea

The application of coupled geological and geochemical studies of oilfields can provide information on the maturity of the source rocks that fed the field, the directions of filling and estimates on the timing of accumulation development. In this ongoing study, detailed three-dimensional compositional mapping of the Ula Field permitted the selection of samples for more detailed molecular analysis, providing data on both source facies and maturity. It was observed from these data that, in addition to the expected lateral variations in chemical composition, vertical variations are also evident. Furthermore, local polar compound concentrations (small tar mats that are chemically relatable to the accumulation) are definable on a stratigraphic basis. Despite these chemical compositional variations and maturity differences, the petroleum column in the Ula Field appears mechanically stable. Indeed, the observed chemical compositional variations and the mechanical properties of the field are consistent with Late Tertiary filling as indicated by maturation modelling of maturity parameters.

Aspects of generation and migration of hydrocarbons from coal-bearing strata of the Hitra formation, Haltenbanken area, offshore Norway

Abstract The Haltenbanken area, situated 150–200 km offshore mid-Norway is a proven gas/condensate and oil producing province with continuing exploration potential. In this structurally and stratigraphically complex area with multiple source rock possibilities, we have examined the Upper Triassic/Lower Jurassic Hitra formation to determine its possible contribution to the hydrocarbon pools in the area. The 400 m thick Hitra formation consists of interbedded shales, siltstones, sandstones and coal seams. In this study conventional cores of the Hitra formation from depths varying from 2.4. and 4.6 km have been examined with the objective to obtain a better understanding of hydrocarbon generation, migration and expulsion processes in these coals as function of variation in facies and maturity. In addition to basic geochemical measurements (TOC, Rock-Eval and GC analysis of C15+ saturated hydrocarbons), microscopic studies (normal and UV reflected light) were carried out to determine the abundance and initial distribution of hydrogen rich macerals including fluorescent mobile phases such as exsudatinities. Geochemical and microscopal evidence was obtained suggesting that some of the interbedded sandstones show indications of impregnation by migrated hydrocarbons. Some bitumen filled micro fractures were observed extending from the coals towards the adjacent sands. A preliminary interpretation of these data is that these fractures represent conduits carrying hydrocarbons from the coal seams towards the sandstones. The role of the kerogen type variations on the amount of hydrocarbons generated and the mechanism and efficiency of hydrocarbon expulsion from the coals is discussed.

Characterisation of kerogens, coals and asphaltenes by quantitative pyrolysis—mass spectrometry

Abstract The addition of synthetic polymer standards to geological samples in order to provide quantitative data on pyrolysis product yields of the latter has been shown to be a valuable tool in pyrolysis-gas chromatography (Py-GC). The applicability of this approach for pyrolysis-mass spectrometry (Py-MS) is the focus of the present study. In general, good agreement was found in comparisons of yields of different compound classes determined by Py-MS with data from Py-GC and elemental analysis. Whilst no response factors were determined semi-quantitative data could readily be obtained for the compounds of interest. Examples are given of how different kerogen types can be distinguished not only on the basis of the relative abundance of different compounds, but also their yields. A practical example of the utility of this approach is provided in a study of compositional variations in petroleum asphaltenes. Here, differences were observed not only between petroleum asphaltenes generated from different source rocks, but also within a single oil reservoir (Ula Field, Norwegian North Sea). Whilst Py-MS suffers from several limitations (lack of information on carbon number distribution, compound-specific response factors, multiple contributions to nominal masses) the method has the advantage over Py-GC of being more rapid and providing data in a more readily analysable form. An example of the manner in which to overcome the problem of multiple contribution to nominal masses is shown through the application of higher resolution mass spectrometry.

A rapid correlation method for petroleum population mapping within individual petroleum reservoirs: applications to petroleum reservoir description

While oil-oil and oil-source rock correlation is a routine part of most exploration programmes, it is only rarely that detailed comparison of oil compositional variations within reservoirs are studied. Although many mechanisms serve to homogenize reservoir fluids, petroleum populations of varied composition can be defined and related to reservoir contiguity and the history of field filling from diverse source rock assemblages. The definition of these heterogeneous petroleum populations, while of great significance to exploration problems related to field filling and definition of prospective sub-traps, is also an under-exploited development tool. Conventional approaches to reservoir definition rely heavily on sedimentological and downhole geophysical analysis of formations coupled with flow testing. Petroleum population mapping may allow direct definition of filling/production conduits within fields if mixing processes can be understood and quantified.

The control of maturity and kerogen type on quantitative analytical pyrolysis data

Abstract Quantitative pyrolysis-gas chromatography has been performed on 96 kerogen samples isolated from 17 wells on the Norwegian Continental shelf. Petrographic and bulk geochemical measurements were also performed on the samples, and a combined data set of 117 variables for each sample was analysed using principal components analysis (PCA). This approach provides an objective and reproducible means of kerogen characterisation, which can be easily automated. In addition to objective kerogen characterisation and facile visualisation of facies and maturity related chemical trends, the method has the potential to allow objective prediction of key geochemical parameters such as maturity level from pyrogram data.

Recognition and quantification of the effects of primary migration in a Jurassic clastic source-rock from the Norwegian continental shelf

A detailed quantitative geochemical study was made of a mature Jurassic clastic source rock sequence in order to attempt to recognize and quantify petroleum migration effects. Two different quantitative approaches were followed to monitor primary migration effects on both the bitumen-1 (e.g. whole rock extract) and the bitumen-2 (e.g. isolated kerogen extract) fractions. These methods were (A) comparisons of TOC-normalized extract yields and quantified molecular compositions of petroleums from both tightly carbonate cemented zones and those from less cemented zones; and (B) comparison of petroleums from shale laminae vs those from interbedded siltstone laminae. On this basis it was possible to assess apparent primary migration efficiency. Relative expulsion efficiencies in certain shale samples and relative import efficiencies in siltstone bands ranged from 11% (total extract) to 85% (saturated hydrocarbons and individual n-alkanes). No carbon number dependent fractionation effects were observed for n-alkanes of bitumen-1 in the C15 to C35 range (i.e. expulsion efficiencies were independent of carbon number) suggesting bulk oil flow with chromatographic modification as the predominant transport mechanism.

Variation in organic matter quality and maturity in Draupne formation source rocks from the Oseberg field region, offshore Norway

Abstract Detailed characterization of the kerogens from closely spaced samples from the Draupne Formation penetrated by two wells, A and B, in the Oseberg Field region has been carried out by geochemical and petrographic methods. For selected samples the C15-hydrocarbon fraction has been examined by GC and GC/MS. Petrographic analyses by transmitted light and UV excitation fluorescence methods demonstrate that the organic matter in all samples from both wells consists predominantly of amorphous material. Total organic carbon contents and hydrogen indices indicate an overall trend of decreasing kerogen quality with increasing depth for the Draupne Formation in well A. Similar data for samples from well B demonstrated generally inferior kerogen quality. Maturation level for the organic matter of the Draupne Formation in well A is slightly higher than that in well B, while both Draupne sections are within the limits for an immature source rock with a type II kerogen. GC- and GC-MS-analysis showed that the samples from well B were enriched in lighter hydrocarbons as compared to samples from well A. It is suggested that this may be due to minor staining of the former samples by migrated light hydrocarbons from the underlying Upper Jurassic sandstone reservoir. A correlation between the two sections of Draupne Formation based on organic facies characteristics indicate a general correspondance between the lowermost part of the section in well A and the section in well B.