Geochemical Logging Research Articles

Organic facies variation from well data on the Cudi Group, the eastern part of SE Turkey

South Turkey is situated on the transition zonebetween the Arabian–African plate and Alpine–Hima-layan orogenic belt. The eastern segment of South Tur-key is located at the northern margin of the Arabianplate (Fig. 1). This is the only tectonostratigraphic unitin Turkey where significant petroleum production istaking place. The region includes rock units, ranging inage from Cambrian to Miocene Derik, and HaburGroup strata are the basement of the autochthonousrock units of Southeast (SE) Turkey (Fig. 2).The Middle Triassic–Lower Jurassic Cudi Group(Figs. 2, 3) units are composed mainly of evaporiticdeposits in the west and marine carbonates in the eastpart of SE Turkey. During the detailed geological stud-ies, the geological structures and the tectonostrati-graphic relations that have developed at different geo-logical periods of the southeastern Anatolia are rela-tively well known and have been documented innumerous publications [1–16]. Also source rock poten-tial has been reported from several sedimentary unitswhich was deposited in the western and eastern part ofSE Turkey [17–34]. By contrast, only a few studieshave reported the organic facies in the Cudi Group. Inthis study, six geochemical logs from Cilesiz-1, B.Yolacan-1, G. Dincer-1, Camurlu-101, Camurlu-102, andYolacan-3 wells, which were drilled by the TurkishPetroleum Corporation in the same area, were used.The purpose of this work is to characterize the organicgeochemistry of the Middle Triassic to the LowerJurassic Cudi Group of the eastern part of SE Turkey, todetermine their kerogen type, and to assess theirorganic facies properties.Southeast Anatolia was situated in the southernhemisphere as a part of the Gondwanaland during thePaleozoic. The Arabian Plate and its northern edge (SEAnatolia) were affected by the epirogenic movementsof Caledonian and Hercynian orogenic events [35–38].All sediments were deposited on the continental shelf

Petroleum geochemical proxies for reservoir engineering parameters

The prediction of fluid flow behaviour in petroleum reservoirs is influenced by the physical and chemical processes active in interacting crude oil/brine/rock systems. It is usually not possible to assess these complex systems directly so proxies for molecular scale behaviour are needed. By their very nature, polar non-hydrocarbons are sensitive to fluid–rock interactions, and if properly exploited they may be utilised as proxies for describing reservoir engineering properties (e.g. wettability) that are also sensitive to fluid–rock interactions. We have identified a group of aromatic oxygen (alkylphenols and alkylfluorenones) and aromatic nitrogen (alkylcarbazoles) compounds present in petroleum that appear to respond to variations in fluid–rock properties. Here we describe the chemical and physical changes in a series of core samples obtained from North Sea reservoirs. A number of petrophysical parameters displayed strong correlations with polar non-hydrocarbon occurrence. For example, deflections in gamma ray logs in response to clay content in a coarsening upwards sandstone unit also showed similar deflections from a number of geochemical logs. A core-flood experiment was designed to monitor the chemical and physical changes during oil migration in a siltstone core. Following completion of the core-flood experiment, Environmental Scanning Electron Microscopy (ESEM) analysis of core samples indicated hydrophilic and hydrophobic surface tendencies grading throughout the core. The distributions of polar non-hydrocarbons (e.g. C 0–C 3–phenols) appear to correspond closely to the observed wettability alteration. The results confirm the potential for developing proxies for fluid–rock interactions through monitoring the surface active compounds present in the polar non-hydrocarbon fraction of petroleum.

Exploration strategies for complex fault block reservoirs in the Subei basin

The geological background of the Subei basin is that of small relief subsidence, low geothermal gradient, multi-sedimentary hiatuses, intense reconstruction of the basin, frequent magmatic activity, and a unique combination of source rock and reservoir. This geological background resulted in characteristics such as many small fault blocks, multiple oil-bearing formations, scattered oil distribution, mini- to small-sized reservoirs, and difficulties in exploration. Aimed at such characteristics, an effective exploration strategy was adopted, and the oil reserves, production and economic benefits of the Jiangsu oilfield were significantly increased. This exploration strategy included understanding the hydrocarbon generation mechanism of source rocks, progressive evaluation of oil resources, comprehensive research on the faulted systems, the distribution of oil reservoirs and their controlling factors. The techniques used included integration of acquisition, processing and interpretation with 3-D seismics as the core technology, trap description and evaluation, directional drilling and cluster drilling, integration of cuttings logging, gas chromatographic logging and geochemical logging, and integration of early reservoir description and progressive exploration and development. This strategy could be guidance for other complex fault blocks.

A procedure for appraisal of a hydrocarbon reservoir continuity and quantification of its heterogeneity

Quantification of the heterogeneity in a hydrocarbon reservoir, which is crucial during different production phases, could be performed using qualitative and semi-quantitative data such as analysis of reservoir sedimentology, stratigraphy, reservoir architecture, measurement of physical and geochemical properties of the hydrocarbons and formation waters. In this study, strontium isotope residual salt analysis (Sr-RSA) method is used to examine possible reservoir compartmentalization in a Permo–Triassic carbonate reservoir, in Persian Gulf. Different Permo–Triassic units in the studied field show heterogeneity in lithology that is reflected in reservoir compartmentalization. Detailed profiles of 87Sr/ 86Sr ratio of residual salts show two main different reservoirs down the studied well, i. e., upper and lower reservoirs. Moreover, the upper reservoir shows some Sr-heterogeneity that indicates small scale compartmentalization. The isotopic composition of formation waters from different levels of reservoir indicates that samples enriched in 18O. δD values of two reservoir unit (K1 and K2 + K3) are higher than the δD values of K4. The last highly D-depleted water sample could indicate that this formation water is affected by isotope exchange with clay minerals and/or by other fluids such as those rich in aqueous or gaseous hydrocarbons. Inferred compartmentalization from these geochemical methods is compared with independent evidences such as geochemical logs and horizontal permeability data which reconfirmed reservoir compartmentalization.

Wire-line log-based stratigraphy of flood basalts from the Lopra-1/1A well, Faroe Islands

The present study shows that it is possible to use conventional borehole logs to perform a detailed lithological/stratigraphical division of a column of subaerially extruded basalt. A stratigraphical division of the subaerial flood basalts penetrated by the Lopra-1/1A well has been carried out using new wire-line logging data measured in 1996 in the interval 200–2489 m depth. Resistivity data acquired in the interval 200–2178 m depth during 1981 after the initial drilling of the Lopra-1 well have also been incorporated. Eighty-six individual flow units, 18 compound flows and two dolerite dykes have been identified by combining the NPHI porosity, RHOB density, P-, S- and Stonely-sonic transit time, calliper and resistivity logs. Fifty-two sedimentary/tuffaceous layers have also been identified using the CGR and SGR gamma ray and potassium logs in combination with the aforementioned logs. Within the flow units, sonic velocity, density and resistivity are highest in the core where porosity is lowest. This relation is reversed in the uppermost and basal zones of the flow units. The sonic velocity in the core seems to be independent of the thickness of the flow unit. Porous zones seem abundant in some cores and the total section of cores containing porous zones constitutes more than 70% of the thickness of its flow unit, but where porous zones are absent the core makes up only roughly 50% of the thickness of the flow. It is suggested that the flow units with porous cores represent aa flows (88% of the flow units) and the others pahoehoe flows (12% of the flow units). The log pattern of the flow units (crust, core and basal zone) is similar to log patterns reported from other basalt plateaux. However the patterns in Lopra-1/1A show a larger variation than elsewhere, suggesting that the flow units are more complex vertically than previously thought. Statistical analysis of P-, S- and Stonely-waves, RHOB, NPHI, resistivity, gamma and calliper logs has been carried out. Cross-plots based on the lithological divisions have been produced that show a pronounced reduction in scattering versus P-sonic transit time and P- and S-sonic, RHOB and NPHI logs correlate with depth. The geochemical logs do not reflect the cyclic structure of the flow units and probably represent the primary composition of the basalt. The thorium log especially indicates flow units with high and low radioactivity and it is suggested that a minimum of 36 flow fields form the logged part of the lower basalt series. Dolerite units described in previous works have been confirmed based on the combined interpretation of wire-line logs. The log data suggest that the subaerially extruded basalt has its base at a depth of approximately 2490 m and that a hyaloclastite succession is found below that depth. The transition from subaerially extruded basalt to hyaloclastite produces a negative acoustic impedance and it is found that the transition corresponds to a negative reflection interpreted on VSP surveys from 1988 and 1994.

The analysis of oil trapped during secondary migration

During secondary migration, there is an opportunity for oil to be trapped as fluid inclusions (FIs) within framework grains such as quartz and within diagenetic cements that have a crystalline structure. Oil saturation on migration pathways remains relatively low, so typically fewer oil inclusions get trapped compared with samples from an oil column. Geochemical analysis of the much smaller amounts of inclusion oil present in samples from interpreted oil migration pathways has been attempted for two samples from the Champagny-1 and Delamere-1 wells in the Vulcan Sub-Basin, northern offshore Australia. A combination of petrographic analysis, bulk geochemical inclusion analysis and log evaluation confirmed that both samples were from oil migration pathways. Despite the small number of oil inclusions, reliable geochemical data were acquired from both samples that were significantly above the levels detected for the system and outside-rinse blanks. The FI oil trapped on the interpreted oil migration pathway in Champagny-1 was generated from clay-rich marine source rock with little terrigenous organic matter input. It was generated at peak oil window maturity and correlates best with oils derived from the Late Jurassic Lower Vulcan Formation. In contrast, the Delamere-1 FI oil contains evidence of greater input of terrigenous organic matter and was generated at early oil window maturity. This FI oil also contains a signature of a biodegraded component, which could have been generated either from the Middle Jurassic Plover Formation, or from an older source rock. These data indicate that it is feasible to geochemically map migration pathways across prospects or basins, and to analyse palaeo-oil compositions in oil zones where the few inclusions get trapped. This also suggests that the few oil inclusions that sometimes occur in Proterozoic or Archaean rocks may be analysable in the future, which would provide relatively pristine and robust data on the composition and diversity of Earth’s early biosphere.

Borehole geophysical techniques to define stratigraphy, alteration and aquifers in basalt

This paper concerns the interpretation of borehole geophysical data from basalt sequences, especially continental basalt sequences that host aquifers. Based on modifications of the rules used for interpreting borehole data from sedimentary rocks, new rules are proposed to identify the internal stratigraphy, aquifer boundaries, and alteration features in continental basalts. The value of several wireline tools is critiqued. Natural gamma logs have limited utility in basalt sequences unless anomalously high-potassium or low-potassium basalt flows and/or sedimentary interbeds exist which can act as marker beds for stratigraphic correlations. Neutron logs can usually discriminate between individual flows, flow breaks and interbeds, even in unsaturated basalts. Neutron logs and temperature logs can also be used to map aquifer thickness in basalt. Gamma–gamma density logs are usually sensitive to the density contrasts between interbeds and basalt flows, and in combination with neutron and natural gamma logs are crucial for the correct interpretation of large void spaces in basalt such as collapsed lava tubes and formerly inflated pahoehoe lobes. Basalt porosity calculated from neutron, resistivity and/or gamma–gamma density logs is commonly overestimated due to the presence of hydrous alteration minerals. Velocity and resistivity logs are best at discriminating between flows in saturated conditions. Magnetic susceptibility logs may capture magnetic mineralogy variations at a finer scale than that of flows and flow breaks and therefore should always be interpreted in combination with other logs. Non-spectral neutron–gamma logs are not useful in basalt, though spectral neutron–gamma logs have been used successfully for stratigraphic correlation and to locate pollutants. Geochemical logs or the inclination of magnetic remanence provide the best data to discriminate individual flows with a basalt sequence, and thus establish an internal stratigraphy. Other tools used alone cannot provide reliable stratigraphic information, but a combination of tools may work. We recommend the combination of natural gamma, neutron, and gamma–gamma density logs in unsaturated rocks, and these logs plus velocity and resistivity logs in saturated rocks.

Evaluation of Petroleum Source Rocks on the Coastal Area of the Western Taurus Region, Turkey

The source-rock potential of the autochthonous Mesozoic carbonates in the western Taurus region of SW Turkey has been investigated. The core samples were obtained from three wells and belong to the Jurassic-Cretaceous Beydoglari units, including Beydagi and Kuyubasi formations. Two different compressional tectonisms influencing the source-rock maturation occurred during Upper Cretaceous and Lower Miocene. The Beydagi formation penetrated by the Demre-I well has a potential source-rock interval that has the capacity to generate oil. The geochemical log of the Kas-1 well shows that the Kuyubasi samples have poor source-richness. In the Kuyubasi-1 well, Rock-Eval pyrolysis data indicate that the Beydagi formation samples have high Tmax and low HI values, corresponding to thermally over-mature and gas-prone type III kerogen. Although surface samples from the Beydagi formation have high total organic carbon (TOC) values, ranging from 7.22 to 9.54% , their low Tmax values (<430°C) indicate that the samples are immature. The mass of petroleum expelled by the source rocks ranges from 2.01 to 46.90 mg Hc/g rock.

Quantitative lithology: open and cased hole application derived from integrated core chemistry and mineralogy database

Abstract A new quantitative lithology interpretation is based on elemental concentrations of silicon, iron, calcium and sulfur available from logs. The lithology interpretation is founded on an integrated chemistry-mineralogy core database comprising over 400 samples from many wells of predominantly sand and shaly sand composition located on four continents. The lithological components include ‘clay’, which is the sum of all clay minerals; ‘carbonate’, which is the sum of calcite and dolomite; ‘anhydrite’, which is the sum of anhydrite plus gypsum; and ‘sand’ or ‘quartz-feldspar-mica’, which is the remainder of the formation essentially constituting the sand fraction. The new interpretation demonstrates that the elements aluminium alone or a combination of silicon, calcium, and iron provide a much more accurate estimation of clay than either gamma ray or its individual components potassium, thorium and uranium. Calcium alone or calcium and magnesium are used to determine carbonate concentrations. Calcium and sulfur can be used to estimate the anhydrite fraction. Having estimated the total clay, carbonate, and anhydrite fractions, the remainder of the formation is assumed to be composed quartz, feldspar, and mica minerals. Examples of the new lithology interpretation are provided for core data and also for geochemical log data from both open and cased hole environments.

The estimation of modal mineralogy: a problem of accuracy in core-log calibration

Abstract In the case study described here the quantitative modal mineralogy of a number of core samples was determined with the objective of using these modes to calibrate geochemical logs. Modal estimates were obtained for the core samples by quantitative X-ray diffraction, infrared spectroscopy, point counting of thin sections, and indirectly by calculation from a complete chemical analysis of the samples. In the case of calculated modes, three different algorithms were applied. A by-product of this particularly complete dataset is the possibility of evaluating the most accurate method of modal analysis, and although no certain conclusion is reached on this point the analysis of these data does demonstrate the difficulty of obtaining accurate modal estimates. The core samples, taken at regular intervals through a sand, sandy-shale sequence, capped by a carbonate unit, have a mineralogy which, although dominated by quartz, includes feldspars, carbonates, and clays (illite, kaolinite) together with minor phases. There was generally good agreement between methods in the estimation of quartz, total carbonate, albite, kaolinite, total clay and pyrite. The results for illite and K-feldspar were poor, a reflection of their relatively low concentrations (&lt;10%), and problems of compositional co-linearity in the calculated modes.

Abstract: Continuous In-Situ Evaluation of Coal Resources and Acid Mine Drainage Potential with Wireline Geochemical Logging

Abstract: Continuous In-Situ Evaluation of Coal Resources and Acid Mine Drainage Potential with Wireline Geochemical Logging

Petrophysical estimation from downhole Mineralogy logs

Abstract A number of physical and chemical properties of rocks, many of which are important in petrophysics are simple, usually linear functions, of a rock’s mineralogy. Examples include matrix density, porosity, magnetic susceptibility and cation exchange capacity. Mineralogy logs can be obtained directly from geochemical logs through an inversion process and, in turn, estimates of petrophysical properties such as those noted above can be obtained. The accuracy of such estimates is directly dependent on the quality of both the inversion and the geochemical measurements. Examples from producing oil fields and from deep-sea environments are used to show that accurate estimates of derived parameters can be obtained provided that appropriate inversion procedures are adopted, and that these estimates are generally better than those obtained by conventional log analysis. Logs of some parameters, such as cation exchange capacity, cannot be obtained except through the mineral inversion.

Abstract :Application of Geochemical Wireline Logs to Lithological Interpretation, Reservoir Description, and Formation Evaluation

Abstract :Application of Geochemical Wireline Logs to Lithological Interpretation, Reservoir Description, and Formation Evaluation

Formation Evaluation of Gas-Hydrate-Bearing Reservoirs with Geochemical Log Measurements : ABSTRACTS

Formation Evaluation of Gas-Hydrate-Bearing Reservoirs with Geochemical Log Measurements : ABSTRACTS

Geochemical Study of Oils and Oil Source Rock from the Eastern Drava and Slavonija-Srijem Depressions, Pannonian Basin, Croatia

A Middle Miocene oil source rock has been identified by geochemical logging of exploration wells in the Eastern Drava depression (EDD) and in the Slavonija-Srijem depression (SSD) at the south-east margin of the Pannonian Basin, Croatia. The source rock contains Type II to II/III kerogen and reaches early maturity stage at depths of about 2400 m. The Mesozoic sequences were found either to be absent or to be poor in organic matter within the analysed wells. According to C27-C28-C29 sterane and C27-C28-C29 monoaromatic steroid distributions, gamacerane indices, C35 homohopane indices and presence of C30 steranrs, the EDD and SSD oils wrre generated from the same marine source rock. The distribution of homohopanes and high Ni/(Ni + V) porphyrin ratios indicates that the related source rock was deposited under suboxic conditions. The oils appear to have been generated from the source rock during the early to middle stages of maturity. The oils are waxy, with pour points up to 30°C and sulphur contents not exceeding 0.7%.

Aptian‐Albian sea level history from Guyots in the western Pacific

Relative sea level fluctuations are an important control on patterns of sedimentation on continental margins and provide a valuable tool for regional correlations. One of the main objectives of combined Ocean Drilling Program Legs 143 and 144 was drilling the thick carbonate caps of a suite of seamounts, called guyots, scattered over the northwestern Pacific. The array of drowned Cretaceous banks includes four carbonate banks of Aptian‐Albian age. These particular carbonate banks display emergent surfaces if regional sea level falls faster than the rate of guyot subsidence, or intervals of condensed parasequences and well‐cemented peritidal crypto‐algal flats if the rate of sea level fall is slightly less than guyot subsidence. Rapid rises of sea level following these sequence boundaries are recorded as drowning of the emergent horizons or as pronounced deepening of facies. The cored lithologies and downhole geophysical and geochemical logs were used to identify depositional sequences and surfaces of exceptional shallowing or deepening. A combination of biostratigraphic datums, carbon and strontium isotope curves, relative magnitude of surfaces of emergence, relative thicknesses of depositional sequences, sea level events, and counts of upward shallowing cycles or parasequences were used to correlate sequences among the four sites. After compensating for thermal subsidence rates at each guyot, an identical pattern of major Aptian‐Albian eustatic sea level events is evident throughout this large portion of the Pacific Ocean. There are approximately 12 Aptian and 12 Albian significant sequence boundaries, of which a third were associated with major episodes of emergence. When these events are compared with Aptian‐Albian relative sea level changes observed in European shelf successions, the major sequence boundaries and transgressive surges can be easily correlated, and it appears that both regions also display the same number of minor events. Therefore we can apply the relative timing of these events from the thermal subsidence rates and parasequence counts of the Pacific banks to construct an improved scaling of the associated ammonite zones and biostratigraphic datums in the Aptian‐Albian interval.An electronic supplement of this material may be obtained on adiskette or via Anonymous FTP from KOSMOS.AGU.ORG (LOGINto AGU's FTP account using ANONYMOUS as the username andGUEST as the password. Go to the right directory by typing APEND.Diskette may be ordered from American Geophysical Union, 2000Florida Ave., N.W., Washington, D.C. 20009, $15.00. Payment mustaccompany order.

Characterization of Soultz hydrochemical system: WELCOM (Well Chemical On-line Monitoring) applied to deepening of GPK-1 borehole: L. Aquilina & M. Brach, Geothermal Science & Technology, 4(4), 1995, pp 239–251

During the second half of 1992, the GPK-1 well at Soultz-sous-Forets (Alsace, France), which is devoted to geothermal Hot Dry Rock research, was deepened from roughly 2000 m to 3600 m. The interval from 2200 to 3600 m was geochemically logged, which included: (1) continuous monitoring of the physico-chemical parameters of drilling fluids (pH, Eh, temperature and conductivity) at the well inflow and outflow; (2) continuous analysis of the drilling fluid gas content (He, Rn, O 2 , CO 2 , CH 4 ) at the well outflow; and (3) discontinuous drilling fluid sampling and analysis at the well inflow and outflow. Daily processing of the physico-chemical parameters and gas-concentration measurements gave real-time information on the fractures intersected-by drilling, distinguishing between: (1) sealed fractures (slight He anomalies); (2) incompletely sealed fractures allowing pervasive gas circulation (slight He and Rn anomalies); (3) fractures with past water circulation (He without Rn and slight chemical changes); (4) fractures with present water circulation (He-Rn-CO 2 -CH 4 anomalies). Formation water mixing with drilling fluids lowered the pH and Eh, and increased the conductivity. Mud samples were collected at a mean rate of one sample (inflow and outflow) every 6 m. On-site analyses of Cl, Ca, SiO 2 and alkalinity were done within 24 hours after sampling. The mixing of water derived from the altered zones with the drilling fluids, is shown by the differences between inflow and outflow from the well. The chemical analyses indicate that mixing of less than 1 m 3 of formation fluid has taken place from 2814 to 2817 m, which shows the very good precision of the method. A water inflow of about 1 m 3 /h of highly concentrated Na-Cl-Ca-type fluids is deduced for the major fracture zone at 3500 m. Geochemical logging has proven to be a very efficient tool for drilling assistance, especially in the case of formation fluid inflow, which disturbs the driller's control of mud quality

Ages and Depositional Environments of Paleogene Rocks in the Southern Maracaibo Basin, Venezuela: ABSTRACT

Detailed studies of Paleogene rocks exposed in road cuts and rivers along the north flank of the Merida Andes in Venezuela provide a means of assisting the interpretation of subsurface data and therefore assessment of the hydrocarbon potential of the Southern Maracaibo Basin. Thick sections at Rio Chama, Rio Lora and Rio Lobaterita, together with many smaller outcrop localities, were measured and sampled to determine the ages and depositional environments of Paleogene formations. Observations and measurements taken at the outcrop include gamma ray signature, bed thickness, mineralogy, and associated sedimentary structures. Extensive sets of samples were taken for paleontological (especially palynology) analyses. This paper highlights the significant geological characteristics of the Barco, Los Cuervos, Mirador, and Carbonera formations, illustrated with measured sections showing gamma ray, geochemical, and palynological abundance logs. These observations together with proprietary subsurface data have made it possible to construct maps of Paleogene paleogeography of the Southern Maracaibo Basin, providing an understanding of the depositional environments and regional extent of potential hydrocarbon reservoirs and seals.

Geophysical and geochemical logs from a copper oxide deposit, Santa Cruz project, Casa Grande, Arizona : P. H. Nelson & D. Johnston, Geophysics, 59(12), 1994, pp 1827–1838

Geophysical and geochemical logs from a copper oxide deposit, Santa Cruz project, Casa Grande, Arizona : P. H. Nelson & D. Johnston, Geophysics, 59(12), 1994, pp 1827–1838

Transformation of Geochemical Log Data to Mineralogy Using Genetic Algorithms: ABSTRACT

Transformation of Geochemical Log Data to Mineralogy Using Genetic Algorithms: ABSTRACT