86 results found
, 1996, Subaerial exposure unconformities on the Vercors carbonate platform (SE France) and their sequence stratigraphic significance, Geological Society Special Publication, Vol: 104, Pages: 295-320, ISSN: 0305-8719
The integration of data on diagenesis and stratal geometry at the margin of the Vercors carbonate platform (SE France) shows that the most prominent break in depositional style does not coincide with the platform-top horizon exhibiting the most extensive meteoric alteration. This observation again illustrates the ambiguity of geometrical criteria to define sequence boundaries related to subaerial exposure. Outcrops at the margin of the Cretaceous Vercors platform expose prograding to aggrading tongues of platform grainstones. Growth and lateral progradation of these platform tongues was frequently interrupted, as evidenced by the deposition of wedges of fine-gmtaed deeper water sediments that encroached the clinoform slopes. Petrographic and geochemical analyses have been carried out at strategic bedding surfaces in order to evaluate the extent to which these breaks relate to sea-level falls and subaerial exposure. The analyses reveal evidence for minor meteoric alteration at all four of the bedding surfaces on the platform top that were studied. However, the Surface 3 bedding plane is unique in that it shows the overprinting of several events of meteoric diagenesis. Petrographic and geochemical analyses suggest that the rudist floatstones at this particular surface were diagenetically overprinted at least three times by meteoric groundwaters that dissolved skeletal grains and precipitated bladed and blocky calcite cements that exhibit bladed to blocky morphologies, low Mg, Mn and Fe abundances, depleted δ18O and δ13C signatures, and freshwater fluid inclusions. Hardground borings then cross-cut the meteoric calcite cements and biomolds, indicating that the subaerial exposure and meteoric overprinting took place prior to deposition of the overlying marine grainstones. This ensuing period of marine inundation was also accompanied by the deposition of red argillaceous internal sediments and dolomitization. The marine grainstones overlying Surface 3 contain
Worden RH, Smalley PC, 1996, H2S-producing reactions in deep carbonate gas reservoirs: Khuff Formation, Abu Dhabi, CHEMICAL GEOLOGY, Vol: 133, Pages: 157-171, ISSN: 0009-2541
Worden RH, Smalley PC, Oxtoby NH, 1996, The effects of thermochemical sulfate reduction upon formation water salinity and oxygen isotopes in carbonate gas reservoirs, GEOCHIMICA ET COSMOCHIMICA ACTA, Vol: 60, Pages: 3925-3931, ISSN: 0016-7037
Smalley PC, Hale NA, 1996, Early identification of reservoir compartmentalization by combining a range of conventional and novel data types, SPE FORMATION EVALUATION, Vol: 11, Pages: 163-169, ISSN: 0885-923X
, 1996, New tools target oil quality sweetspots in viscous oil accumulations, Pages: 911-917
API gravity and dead oil viscosity can be predicted using geochemical parameters measured from core or sidewall core samples. Spatial variations in oil properties can be difficult to map empirically simply by contouring data, since more than one controlling factor may be operative. A major step in improving fluid mapping is to identify and quantify these controls and map them. These maps are then used to attempt oil sweetspot prediction in areas away from well control.
, 1995, Compositional heterogeneities in oilfield formation waters: Identifying them, using them, Geological Society Special Publication, Vol: 86, Pages: 59-69, ISSN: 0305-8719
Two new techniques are now available for gaining information on formation water composition from core samples: core centrifugation, in which preserved core samples, complete with their preserved fluid content, are ultracentrifuged to extract the oil and water; and residual salt analysis (RSA), where salts are redissolved from unpreserved dry core and parameters such as 87Sr/86Sr are measured in the leachate. These methods are described and details given on the quality control measures that are crucial in order to avoid artefacts of core contamination by drilling fluids. Such methods have allowed formation-water compositional data to be obtained at the sub-metre scale, in some cases revealing significant small-scale variations (gradients and steps) in formation-water chemistry. Resistivity (related to salinity) variations are important for interpretation of water saturation from electrical resistivity logs, which in turn affects the estimate of the total amount of oil contained in the field. In the Machar chalk-reservoired oilfield, formation-water data derived by RSA and core centrifugation both indicate variations in salinity (TDS 140 000-220 000 ppm) that significantly affect resistivity. In some oilfields, assumption of a single resistivity value for the whole field could thus lead to errors in petroleum reserves estimation. Step changes in formation-water composition can be used as indicators of reservoir fluid compartmentalization, important for field development strategy. Examples are given of cases where RSA variations have been used to distinguish between laterally restricted and laterally extensive shales, both in the oil and water legs. Analysis of water compositional variation forms a valuable addition to the range of tools available for assessing reservoir compartmentalization during reservoir appraisal. This is because natural variations in fluid compositions provide quasi-dynamic data (i.e. how fluids have moved slowly during geological time), from whi
, 1995, Toolkit for early identification of reservoir compartmentalization, Pages: 25-34
Reservoir compartmentalization (vertical and lateral) is often a major uncertainty at the field appraisal stage, impacting important investment decisions. Unfortunately the most definitive compartmentalization data (dynamic production data) are not usually available so early in field life. This paper illustrates how early indications of compartmentalization can be achieved by integration of various types of static data, using an example from the Ross oilfield, UKCS. Here, integration of oil molecular maturity parameters, GC fingerprinting, PVT data, well test analysis and fault seal analysis with interpretation of 3-D seismic revealed the presence of several sealing faults, and thus enabled the field to be described in terms of its natural segmentation. Vertical compartmentalization was investigated using a combination of oil geochemistry (maturity, GC fingerprinting), residual salt analysis and high-resolution stratigraphy, revealing a continuous shale that has a lateral extent of >5 km; it divides the reservoir into two main layers. The results of this study helped to decrease uncertainty in field development planning. The key message is that there are many tools available in the toolkit to aid in assessing compartmentalization, but individually they each only give part of the picture. It is therefore important to use several tools in combination.
WORDEN RH, SMALLEY PC, OXTOBY NH, 1995, GAS SOURING BY THERMOCHEMICAL SULFATE REDUCTION AT 140-DEGREES-C, AAPG BULLETIN-AMERICAN ASSOCIATION OF PETROLEUM GEOLOGISTS, Vol: 79, Pages: 854-863, ISSN: 0149-1423
, 1995, Diagenesis of the Machar Field (British North Sea) chalk: evidence for decoupling of diagenesis in fractures and the host rock, Journal of Sedimentary Research A: Sedimentary Petrology & Processes, Pages: 105-111
The present-day formation waters of the Machar Field have87Sr/86Sr ratios significantly higher than the whole rock and fracture-filling cement calcite values, evidence that the chemical composition of the formation waters is not representative of that of the pore waters during chalk re-crystallization. Little diagenesis is therefore now occurring in the Machar Field. The bulk of calcite cementation in fractures and the recrystallization and cementation of the chalk matrix occurred at temperatures in the 80-100°C range, at or just below the present-day reservoir temperature of 97°C. -from Authors
WORDEN RH, WARREN EA, SMALLEY PC, et al., 1995, EVIDENCE FOR RESETTING OF FLUID INCLUSION TEMPERATURES FROM QUARTZ CEMENTS IN OILFIELDS - DISCUSSION, MARINE AND PETROLEUM GEOLOGY, Vol: 12, Pages: 566-570, ISSN: 0264-8172
Richard H Worden PCS, 1995, Reactions Controlling H2S Concentration in Deep Carbonate Reservoirs: ABSTRACT, AAPG Bulletin, Vol: 79, ISSN: 0149-1423
, 1994, The Amethyst Field, Geological Society Memoir, Vol: 15, Pages: 3-77, ISSN: 0435-4052
Warren EA, Smalley PC, 1994, North Sea formation waters atlas, ISBN: 1897799071
This atlas provides data on formation water compositions from throughout the North Sea Basin. The objective was not simply to provide a table of water compositions, but to supply as much supplementary data on reservoir conditions and geological setting as possible to enable the individual data to be placed in context. The atlas contains four main sections: a field compendium, comprising a field-by-field compilation of water compositions with accompanying geological description and field maps; a water resistivity (R w ) atlas; scale prediction maps estimating susceptibility of produced waters to scaling; and a water chemistry atlas. -from Editors
, 1994, Part 2: SPWLA Water Resistivity (R<inf>w</inf>) Atlas, Geological Society Memoir, Vol: 15, Pages: 79-103, ISSN: 0435-4052
, 1994, Reservoir compartmentalization assessed with fluid compositional data, SPE Reservoir Engineering (Society of Petroleum Engineers), Vol: 9, Pages: 175-180, ISSN: 0885-9248
Fluid composition is a valuable addition to the battery of 'static' data available during reservoir appraisal than can be used to predict the dynamic behavior of the reservoir later in field life. This is because fluid data are not truly static; natural fluid mixing is a dynamic process that occurs over a long (geologic) time scale. Oil compositional differences, especially those that parallel changes in density, should be mixed rapidly by convection; their preservation indicates barriers to fluid flow. Water variations, now measurable on conventional core samples by use of residual salt analysis (RSA), help identify barriers to vertical fluid flow in oil and water legs.
SMALLEY PC, ENGLAND WA, 1994, RESERVOIR COMPARTMENTALIZATION ASSESSED WITH FLUID COMPOSITIONAL DATA, SPE RESERVOIR ENGINEERING, Vol: 9, Pages: 175-180, ISSN: 0885-9248
SMALLEY PC, HIGGINS AC, HOWARTH RJ, et al., 1994, SEAWATER SR ISOTOPE VARIATIONS THROUGH TIME - A PROCEDURE FOR CONSTRUCTING A REFERENCE CURVE TO DATE AND CORRELATE MARINE SEDIMENTARY-ROCKS, GEOLOGY, Vol: 22, Pages: 431-434, ISSN: 0091-7613
SMALLEY PC, BISHOP PK, DICKSON JAD, et al., 1994, WATER-ROCK INTERACTION DURING METEORIC FLUSHING OF A LIMESTONE - IMPLICATIONS FOR POROSITY DEVELOPMENT IN KARSTIFIED PETROLEUM RESERVOIRS, JOURNAL OF SEDIMENTARY RESEARCH SECTION A-SEDIMENTARY PETROLOGY AND PROCESSES, Vol: 64, Pages: 180-189, ISSN: 1073-130X
HEAMAN LM, SMALLEY PC, 1994, A U-PB STUDY OF THE MORKHEIA COMPLEX AND ASSOCIATED GNEISSES, SOUTHERN NORWAY - IMPLICATIONS FOR DISTURBED RB-SR SYSTEMS AND FOR THE TEMPORAL EVOLUTION OF MESOPROTEROZOIC MAGMATISM IN LAURENTIA, GEOCHIMICA ET COSMOCHIMICA ACTA, Vol: 58, Pages: 1899-1911, ISSN: 0016-7037
BISHOP PK, SMALLEY PC, EMERY D, et al., 1994, STRONTIUM ISOTOPES AS INDICATORS OF THE DISSOLVING PHASE IN A CARBONATE AQUIFER - IMPLICATIONS FOR C-14 DATING OF GROUNDWATER, JOURNAL OF HYDROLOGY, Vol: 154, Pages: 301-321, ISSN: 0022-1694
SMALLEY PC, WARREN EA, 1994, NORTH-SEA FORMATION WATERS - IMPLICATIONS FOR DIAGENESIS AND PRODUCTION CHEMISTRY, MARINE AND PETROLEUM GEOLOGY, Vol: 11, Pages: 2-4, ISSN: 0264-8172
Smalley PC, Bishop PK, Dickson JAD, et al., 1994, Water-rock interaction during meteoric flushing of a limestone: implications for porosity development in karstified petroleum reservoirs, Journal of Sedimentary Research A: Sedimentary Petrology & Processes, Vol: 64 A, Pages: 180-189
The Lincolnshire Limestone, comprising a succession of Jurassic wackestones, packstones, and oolitic grainstones, forms an important carbonate aquifer in eastern England. Meteoric waters enter at outcrop and penetrate between confining strata at least 25 km down-dip. This water dissolves and interacts with the limestone, and even water samples collected at or near outcrop are calcite-saturated. Net limestone dissolution is thus a process that is most dominant in the near-surface environment. The Lincolnshire Limestone may be used as an analogue for karstified petroleum reservoirs, specifically those which have been buried and lost their unstable carbonate minerals (aragonite, high-Mg calcite) prior to uplift and karstification. -from Authors
, 1994, Seawater Sr isotope variations through time: a procedure for constructing a reference curve to date and correlate marine sedimentary rocks, Geology, Vol: 22, Pages: 431-434, ISSN: 0091-7613
A new curve of seawater 87Sr/86Sr variation through the past ~450 m.y. enables dating and correlation of marine sedimentary rocks with quantifiable uncertainties The paper discusses the method by which this curve was constructed. Strictly standardized 87Sr/86Sr analyses of marine were input into a data base. Objective criteria were used to assign each data point a reliability score. These scores were used as initial weights in a robust nonparametric regression (LOWESS) to obtain a curve of seawater 87Sr/86Sr variation with rigorous 95% confidence intervals for prediction of age uncertainty. Precise dating and stratigraphic correlation are feasible for much of the Phanerozoic. -from Authors
, 1994, North Sea formation waters atlas, North Sea formation waters atlas
This atlas provides data on formation water compositions from throughout the North Sea Basin. The objective was not simply to provide a table of water compositions, but to supply as much supplementary data on reservoir conditions and geological setting as possible to enable the individual data to be placed in context. The atlas contains four main sections: a field compendium, comprising a field-by-field compilation of water compositions with accompanying geological description and field maps; a water resistivity (Rw) atlas; scale prediction maps estimating susceptibility of produced waters to scaling; and a water chemistry atlas. -from Editors
Warren EA, Smalley CP, Howarth RJ, 1994, Part 4: Compositional variations of North Sea formation waters, Geological Society, London, Memoirs, Vol: 15, Pages: 119-208, ISSN: 0435-4052
Warren EA, Smalley PC, 1993, The chemical composition of North Sea formation waters: A review of their heterogeneity and potential applications, Pages: 1347-1352, ISSN: 2047-9921
Significant variations in the salinity, chemical and stable isotopic composition of formation waters can be observed in the existing published data from the North Sea Basin. Variations occur at all scales, from intra-formational and within-field to basin-wide variations in water chemistry within and between formations. Clearly, the present-day North Sea Basin waters are highly heterogeneous. These variations have important implications. Various aspects of formation-water chemistry impact upon reserves estimates (/?w) and scale prediction (e.g. Ba, Sr, S04), and also yield valuable information on the processes of water-rock interaction and diagenesis leading to modification of reservoir quality. However, present understanding of the processes causing the compositional heterogeneities is limited by the sparse coverage of the existing published data. Consequently, a new effort is being made to compile formation-water data for all fields in the greatest detail possible. Interim results from this compilation are reported. © 1993 Petroleum Geology '86 Ltd.
, 1993, Prediction of large-scale communication in the Smørbukk fields from strontium fingerprinting, Pages: 1421-1432, ISSN: 2047-9921
This paper discusses a new method which uses intra-field variability of formation water chemistry as a measure of degree of reservoir compartmentalization. If two reservoir units are in good flow communication, it is more likely that any water compositional variations that might have arisen would have homogenized by mixing processes (diffusion, dispersion, flow). On the other hand, a lack of flow communication is likely to inhibit water mixing and thus preserve variations in water compositions. Thus, whatever the cause of intra-field variations in water composition, two reservoir units that have a similar water chemistry are more likely to be in good flow communication than two units with different water compositions. In this study the isotopic composition of strontium (87Sr/86Sr) dissolved in the formation water was used to monitor formation water compositional variability. This natural isotopic tracer is particularly useful as it can be measured simply from conventional core samples by extracting residual salts which have precipitated in the pore spaces as a result of formation water evaporation during storage. The Smerbukk fields (Smorbukk North and Smorbukk South, Haltenbanken area, Norwegian Continental Shelf) comprise three Jurassic reservoir intervals, the Garn, He and Tilje formations. In Smorbukk North the main problems are vertical compartmentalization by areally extensive shales and lateral compartmentalization by extensive quartz cementation. In one well, residual salt analysis (RSA) shows that water 87Sr/86Sr compositions are extremely homogeneous in each of the Tilje 1 and 2 reservoir zones, indicating good internal vertical communication. However, the Tilje 1 and 2 waters are significantly different from each other, indicating that the shale that separates Tilje 1 and 2 is an effective barrier to vertical communication. A similar relation is seen in a second well, suggesting that the shale barrier is indeed laterally extensive. In the second well, Til
EMERY D, SMALLEY PC, OXTOBY NH, 1993, SYNCHRONOUS OIL MIGRATION AND CEMENTATION IN SANDSTONE RESERVOIRS DEMONSTRATED BY QUANTITATIVE DESCRIPTION OF DIAGENESIS, PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY OF LONDON SERIES A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, Vol: 344, Pages: 115-125, ISSN: 0962-8428
, 1992, Deterministic method for assessing reservoir communication based on strontium fingerprinting, Pages: 451-464
Incomplete or ambiguous knowledge about communication is an important area of uncertainty in reservoir simulation. We document several cases in which a new quantitative and deterministic method, strontium fingerprinting, can be used to give exact predictions of reservoir communication and flow units. In all cases, the isotopic composition of strontium (87Sr/86Sr) dissolved in the formation water was used to monitor compositional variability of formation water. This natural isotopic tracer is particularly useful as it can be measured simply from conventional core samples by extracting residual salts which have precipitated in the pore spaces as a result of formation water evaporation during storage. We have termed this technique `Residual Salt Analysis' (RSA). It enables detailed sampling in any part of a well for which there is core available. So far, RSA data have tended to yield rather ambiguous predictions. A quantitative method for treating the data has been lacking, as well as an empirical foundation of interpretations. However, we have tested the following hypotheses with empirical data: Intrafield variability of formation water chemistry may be closely correlated with reservoir compartmentalization. If two reservoir units are in good flow communication, their formation waters are likely to have been homogenized by flow and diffusion. On the other hand, a lack of flow communication will inhibit water mixing and thus preserve variations in water compositions. Thus, whatever the cause of intra-field variations in water composition, two reservoir units that have a similar water chemistry are more likely to be in good flow communication than two units with different water compositions. The Sr-ratio values obtained from five wells (n = 117) show a natural tendency to clustering. This contrasts with, for instance, a statistical normal distribution of the data. Moreover, data from distinct reservoir units in different wells are aggregated as distinct clusters in near
, 1992, Assessing reservoir compartmentalization during field appraisal: How geochemistry can help, Pages: 423-431
This paper describes how deterministic information about reservoir compartmentalization can be derived from the study of variations in reservoir fluid composition (oil and water). Oil compositional differences (e.g. PVT data, GC fingerprints) between-well, especially those that relate to changes in density, are useful indicators of reservoir compartmentalization. Such differences would be rapidly mixed by density-driven convection; their preservation indicates a barrier to fluid flow. Where suitable samples are available (e.g. core extracts, multiple DSTs), oil variations within-well would help identify barriers to vertical flow. Water variations are particularly useful if examined on the intra-well scale, e.g. by using residual salt analysis of strontium isotopes (sensitive indicators of water composition) from core samples. Sr variations in water and oil legs reflect the ability of waters to mix and the detailed oil-filling history. Both help identify barriers to vertical fluid flow. The integration of such deterministic data with other measurements (e.g. pressure, sequence stratigraphy), and with stochastic modelling, will provide the ultimate reservoir description.
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