API Units Research Articles

Application of maximum likelihood and model-based seismic inversion techniques: a case study from K-G basin, India

AbstractSeismic inversion is a geophysical technique used to estimate subsurface rock properties from seismic reflection data. Seismic data has band-limited nature and contains generally 10–80 Hz frequency hence seismic inversion combines well log information along with seismic data to extract high-resolution subsurface acoustic impedance which contains low as well as high frequencies. This rock property is used to extract qualitative as well as quantitative information of subsurface that can be analyzed to enhance geological as well as geophysical interpretation. The interpretations of extracted properties are more meaningful and provide more detailed information of the subsurface as compared to the traditional seismic data interpretation. The present study focused on the analysis of well log data as well as seismic data of the KG basin to find the prospective zone. Petrophysical parameters such as effective porosity, water saturation, hydrocarbon saturation, and several other parameters were calculated using the available well log data. Low Gamma-ray value, high resistivity, and cross-over between neutron and density logs indicated the presence of gas-bearing zones in the KG basin. Three main hydrocarbon-bearing zones are identified with an average Gamma-ray value of 50 API units at the depth range of (1918–1960 m), 58 API units (2116–2136 m), and 66 API units (2221–2245 m). The average resistivity is found to be 17 Ohm-m, 10 Ohm-m, and 12 Ohm-m and average porosity is 15%, 15%, and 14% of zone 1, zone 2, and zone 3 respectively. The analysis of petrophysical parameters and different cross-plots showed that the reservoir rock is of sandstone with shale as a seal rock. On the other hand, two types of seismic inversion namely Maximum Likelihood and Model-based seismic inversion are used to estimate subsurface acoustic impedance. The inverted section is interpreted as two anomalous zones with very low impedance ranging from 1800 m/s*g/cc to 6000 m/s*g/cc which is quite low and indicates the presence of loose formation.

Reservoir characterization of fairways in a tight light oil play of the Upper Cretaceous Cardium Formation, west Pembina, Alberta, Canada

Shallow-marine reservoirs of the Upper Cretaceous Cardium Formation contain one of the largest light oil accumulations in Canada, which have been exploited since the 1950s. Since 2008, multistage fractured horizontal wells have revived this play. This study presents a reservoir characterization of the offshore to shoreface unconventional reservoirs of the Cardium Formation in the halo play along the western margin of the legacy Pembina oil pool in central Alberta. Although permeability of these facies is too low to achieve economic production from vertical wells, they were the main carrier beds forming the migration pathway for charging the updip conventional reservoirs in the Pembina field. Three reservoir facies are present in the west Pembina study area: bioturbated muddy sandstones, interbedded sandstones and mudstones, and hummocky cross-stratified sandstones. Shoreface sandstones are relatively thin in the halo play area; thus, significant production in the horizontal wells is attributed to the thin sandstone beds and bioturbated muddy sandstone reservoir facies. To characterize these unconventional, heterogenous reservoirs, traditional reservoir evaluation techniques are integrated with computerized tomography scanning to better evaluate reservoir properties and flow pathways. Cutoffs of 90 gamma-ray API units, 6% porosity, and 0.075-md permeability were used to create a series of reservoir property maps. These detailed maps show the most prospective area of the halo play is a 4-km (2.5-mi)-wide zone immediately to the west of the legacy west Pembina Pool boundary. This lateral heterogeneity with narrow sweet spot fairways explains the significant production variability in these shallow-marine reservoirs.

Gamma-Ray-Tool Characterization Avoids Traditional Limitations

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 191717, “Is API Enough for Gamma Ray Logs, or Do We Need More?” by Feyzi•Inanc, SPE, and Andreas Vogt, SPE, Baker Hughes, a GE Company, prepared for the 2018 SPE Annual Technical Conference and Exhibition, Dallas, 24–26 September. The paper has not been peer reviewed. A major issue with gamma-ray (GR) logs is that the API definition is valid only if the tool is run in a 4.89-in. borehole filled with fresh water. Although all GR tools are intended to provide the same results for such a well, in the field there is no single one-size-fits-all concept. In this paper, the authors discuss the characterization process for GR tools and how they behave in boreholes different from the one used in the University of Houston (UH) GR characterization pit. Proposals for developing correction strategies so that GR logs become quantitative logs, rather than the qualitative logs of the past, also are outlined. Introduction One of the issues inherent in ensuring repeatability of GR logs starts with tool characterization. For decades, no common industry standard existed that defined the scale for measuring radioactivity. The chaos created by different tools providing radioactivity in different units in the same environments ended with the creation of the API unit in 1959. The definition of the unit was based on work done with the UH GR characterization pit. This specific pit has a 4.89-in. borehole and a casing to ensure stability within the surrounding synthetic formation. One downside of the API definition is that different combinations of radioactive isotopes, environmental conditions, and tool designs may yield comparable count rates. UH GR pit characterizations of GR tools lose their validity as the wellbore diverges from this previously described specific borehole size and environment. Thus, environmental corrections are needed to bring readings to a reference borehole used for tool characterization. Another issue with the UH GR-pit-based characterizations became evident with the emergence of logging-while-drilling (LWD) GR tools. The LWD tools were supposed to provide natural GR values comparable with those of wireline tools. Recommended practices for wireline tools did not have a provision for calibration of LWD GR tools that did not fit into the UH GR pit. Consequently, characterizations of LWD tools to provide results comparable with those of wireline tools became a topic of interest. Background Although the UH GR characterization pit still exists, it suffers from many issues. The facility is in a state of disrepair, with significant corrosion. In addition, its activity level is decreasing because of the presence of short-lived Ra-226 as one of the main ingredients in the high-activity zone. The radionuclide distribution is not even, causing gamma readings to change with the height, and rock dimensions do not correspond to those of an infinite rock environment.

Pseudodensity Log Generation by Use of Artificial Neural Networks

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 180439, “Pseudodensity-Log Generation by Use of Artificial Neural Networks,” by Wennan Long, University of Southern California; Di Chai, University of Kansas; and Fred Aminzadeh, University of Southern California, prepared for the 2016 SPE Western Regional Meeting, Anchorage, 23–26 May. The paper has not been peer reviewed. The challenges of reservoir characterization can be overcome accurately and efficiently by the use of computer-based intelligence methods such as neural networks, fuzzy logic, and genetic algorithms. This paper will describe how one integrates a comprehensive methodology of data-mining techniques and artificial neural networks (ANNs) in reservoir-petrophysical-properties prediction and regeneration. Introduction ANNs—machine-learning models that provide the potential to establish multidimensional, nonlinear, and complex models—can be powerful tools with which to analyze experimental, industrial, and field data. It is crucial to find the optimal data from one well to build the model with ANNs for pseudowell-log generation of a target well. Manual stratigraphic interpretation, though labor-intensive, is regarded as one approach. Data-mining techniques are another applicable approach, involving the automatic processing of data associated with nonlinearity by use of a statistical method to discover data patterns. One application in petrophysics is facies (or electrofacies) classification, which is widely used to divide well-log data to obtain target information. Clustering analysis, an adjunct to artificial intelligence, can determine electrofacies and categorize lithological profiles quite efficiently. Porosity, one of the more important petrophysical properties, can be obtained from density logs. In this study, a three-step approach was produced. First, the authors apply preprocessing of the log data by use of standardization and dimension reduction [ principal-component analysis (PCA)]. Second, they apply clustering [model-based clustering (MBC)] to recognize specific patterns and interpret stratigraphic information. Finally, a similar pattern is chosen as input to generate a target pseudodensity log by use of ANNs. Well-Log-Data Preprocessing Normalization. Well-log data are constructed in a matrix form whereby each row represents the depth record and each column is the different type of well log. Each well constructs one well-log matrix, and one field that has multiple wells constructs a big data set. The initial step in the first stage is to normalize well-log data. This normalization is necessary because different types of well-log data have different units. For instance, the spontaneous-potential log is given in millivolts, whereas the gamma ray log is given in an API unit.

Helical Oligopeptides of a Quaternized Amino Acid with Tunable Chiral-Induction Ability and an Anomalous pH Response.

A series of octamer (8-mer) and hexadecamer (16-mer) oligopeptides of 4-aminopiperidine-4-carboxylic acid (Api) with l-leucine as a chiral auxiliary at their N or C termini were synthesized. By using circular dichroism spectroscopy, the conformational profiles of the peptides were systematically studied, which revealed that the α-helix-formation ability of the peptides is determined by the combination of parameters, which includes peptide length, state of the piperidine groups in the Api units, and position of the chiral auxiliary. When the piperidines were in the free-base state, the peptides showed a low propensity to form helical structures. However, the protonation and acylation of the piperidines enhanced the formation of helical structures, such that the order for helix-formation ability was protonated>acylated>free base. In terms of peptide length, the 16-mers generally showed higher helix-formation ability than the corresponding 8-mers, and one of the 16-mers showed helicity at the highest level reported thus far for oligopeptides of a similar length. It was also found that the sensitivity of the helical structure towards the state of the piperidine groups changed drastically depending on the chiral auxiliary position; the N-terminal chiral peptides were more sensitive than the C-terminal chiral analogues.

A Monte Carlo modeling alternative for the API Gamma Ray Calibration Facility

The gamma ray pit at the API Calibration Facility, located on the University of Houston campus, defines the API unit for natural gamma ray logs used throughout the petroleum logging industry. Future use of the facility is uncertain. An alternative method is proposed to preserve the gamma ray API unit definition as an industry standard by using Monte Carlo modeling to obtain accurate counting rate-to-API unit conversion factors for gross-counting and spectral gamma ray tool designs.

OPTIMIZATION OF MEASUREMENT SPEED FOR SPECTRAL GAMMA RAY AND CLAY MINERAL IDENTIFICATION

Gamma ray log is a logging tool to capture the radioactive level of a rock or formation measured in API units. This logging tool generally has a capability to differentiate between permeable and impermeable layers. Usually the impermeable layer tends to have higher radioactivity compared to the permeable one except for the feldspar bearing formation. In addition, another capability of this logging tool is to determine the kind of clay mineral, by using ratio data between Thorium and Potassium. This laboratory experiment used Spectral Gamma Ray (SGR) equipment at LEMIGAS Routine Core Laboratory. The quality of gamma ray log measurement is significantly affected by the speed of the conveyor belt. During the experiment, the measurement speed of 30 m/hour is the optimum speed to achieve good quality data and time efficiency with the data amount of 169 points/meter. The result of SGR measurement gives the reading on the content of Uranium, Thorium and Potassium. The Thorium and Potassium content are compared and plotted in Quirein graphic which was modified by Schlumberger in 1985. Using this crossplot, we can identify the presence of Chlorite, Montmorillonite, Kaolinite, Illite, mixed layer Feldspar, Mica, and Glauconite minerals. A case study conducted on Wells A1, A2, A3 and A4, indicated that the result of this crossplot were similar to the measurement using XRD.

Log-derived thickness and porosity of the Barnett Shale, Fort Worth basin, Texas: Implications for assessment of gas shale resources

This study estimates reservoir quality and free-gas storage capacity of the Barnett Shale in the main natural-gas producing area of the Fort Worth basin by mapping log-derived thickness, porosity, and porosity-feet. In the Barnett Shale, the density porosity (DPHI) log curve is a very useful tool to quantitatively assess shale gas resources, and gamma-ray (GR) and neutron porosity log curves are important factors in identifying the shale gas reservoir. The key data were digital logs from 146 wells selected based on the availability of GR and density log curves, log quality, and good spatial distribution. The Barnett Shale pay zone was determined on the basis of (1) DPHI >5%, (2) high GR values (commonly >∼90 API units), (3) no significant intercalated carbonate-rich beds, and (4) individual pay zones being thick enough to be commercially successful for the current design of horizontal wells. In the study area, the Barnett Shale pay zone varies from about 165 ft (50 m) to 420 ft (128 m) in thickness (H). Average DPHI values of individual wells for the pay zone vary from 8.5 to 14.0%. Porosity-feet maps of the pay zone show that areas of high DPHI-H values coincide with areas of high natural gas production, indicating that log-derived porosity-feet maps are a good method for evaluating reservoir quality and assessing natural gas resource in the Barnett Shale play. A limitation to this method is shown in the northwestern corner of the study area, which is located in the liquids-rich window with lower thermal maturity.

Experimental and computational investigation of polyacrylonitrile ultrafiltration membrane for industrial oily wastewater treatment

An experimental study on separation of industrial oil from oily wastewater has been done. A polyacrylonitrile membrane with a molecular weight cut-off (MWCO) of 20 kDa was used and an outlet wastewater of API unit of Tehran refinery was employed. The main purpose of this study was to develop a support vector machine model for permeation flux decline and fouling resistance in a cross-flow hydrophilic polyacrylonitrile membrane during ultrafiltration. The operating conditions which have been applied to develop a support vector machine model were transmembrane pressure (TMP), operating temperature, cross flow velocity (CFV), pH values of oily wastewater, permeation flux decline and fouling resistance. The testing results obtained by the support vector machine models are in very good agreement with experimental data. The calculated squared correlation coefficients for permeation flux decline and fouling resistance were both 0.99. Based on the results, the support vector machine proved to be a reliable accurate estimation method.

Asymmetric polyethersulfone ultrafiltration membranes for oily wastewater treatment: Synthesis, characterization, ANFIS modeling, and performance

Oily wastewater in Tehran Oil Refining Company has become a big issue of water and environment pollution to be solved urgently. In this research, polyethersulfone ultrafiltration hollow fiber membranes were prepared via phase inversion induced by dry-jet wet spinning process. The membranes were characterized by water contact measurements, SEM analysis, filtration measurements (flux and fouling). In this work, the results of an experimental study on separation of oil from oily wastewaters are presented. To do these experiments, the outlet wastewater of the API unit of Tehran Refining was used as the feed. The results demonstrated that the prepared polyethersulfone hollow fiber membranes are effective in removing chemical oxygen demand (COD), total organic carbon (TOC), turbidity, total suspended solids (TSS), and oil and grease content (OGC). The results showed that the prepared polyethersulfone hollow fiber membranes exhibit COD, TOC, and OGC removals of about 83.1%, 96.3%, and 99.7%, and final flux and fouling of about 84.1L/(m2h) and 63.0%, respectively. Comparison of results of this method showed that quality of the finally treated outlet water is high and even better than standard water that are currently introduced to cooling towers. Moreover, this paper explores the application of the ANFIS as a model to predict permeation flux in UF process. The selected experimental data were trained by a hybrid learning algorithm combining the forward pass and the backward pass. The good agreement of the model with experimental data underscores the ease and accuracy of the ANFIS in modeling such a complex systems.

Lithofacies, diagenesis and reservoir quality of Upper Shu’aiba reservoirs in northwestern Oman

ABSTRACTCore and log data from Lower Cretaceous limestones of the Upper Shu’aiba Member were used to characterize the distribution of lithofacies, clay and porosity within two low-angle clinoforms that form the reservoirs for an oilfield of northwestern Oman. Data from 15 vertical wells, including four with core, and four horizontal well cores were projected into a dip-oriented cross-section derived from a static reservoir model as a basis for visualizing the above variations. Each clinoform consists of a basal “argillaceous zone” and a thicker “reservoir zone” of clean limestone, together reflecting fourth-order cycles of progradation along the margin of the Bab intra-shelf basin. Lithofacies vary in a proximal direction from mudstone and wackestone (mid-ramp) to mud-dominated packstone (slope) to mud-rich floatstone, rudstone and boundstone (ramp crest) and are arranged in a pattern of decreasing water depth and increasing energy both upwards and landward within each clinoform. In contrast, the reservoir zone of a younger clinoform from a nearby oilfield consists of well-sorted grainstone and grain-dominated packstone, illustrating the wide range of depositional conditions that occurred in the ramp-crest facies belt of different units.Except within the proximal extent of the younger clinoform, where values are transitional toward reservoir zone values, the argillaceous zones have total porosity mostly < 10% and baseline-normalized gamma-ray (GR) activity > 23 API units, reflecting clay contents of around 10–18%. In contrast, most parts of the reservoir zones have GR of 15–23 API units and porosity of 10–35%. Higher clay content is suggested to be linked with lower porosity through facilitation of both mechanical and chemical compaction, the latter providing a local supply of calcite cement. XRD analyses show that the clays are kaolin, illite/smectite and illite, similar to the clays in the overlying Nahr Umr shale. Most former macropores have been filled by blocky calcite cement in the main oilfield studied, but all lithofacies have similar wide ranges of total porosity of 8% to > 30%.The cores were also studied for evidence of diagenesis related to the contact with the overlying Nahr Umr Formation, but profiles of stable-isotope ratios, bulk-rock strontium, petrography and porosity-permeability data show no trends indicative of upward-increasing meteoric diagenesis below this sequence boundary. Meteoric leaching could nevertheless be pervasive throughout the Upper Shu’aiba reservoirs, at least partially accounting for extensive aragonite dissolution and low Sr and δ18O values. Two of the cores show trends of upwards-increasing bulk-rock uranium, manganese and iron, possibly indicative of sea-floor authigenesis. In addition, saddle dolomite near the tops of these cores may reflect late influx of magnesium derived from clay in the Nahr Umr Formation.

Experimental investigation of oily wastewater treatment using combined membrane systems

Investigations were carried out for purification of oily wastewater by a combined of ultrafiltration/reverse osmosis (UF/RO) processes. Laboratory-scale UF using polysulfone (PS) and polyacrylonitrile (PAN) membranes were employed with typical oily wastewater collected from API unit of Tehran refinery. The PAN membrane showed higher rejection, more permeation flux and less fouling resistance than the PS membrane. Both membranes produced permeate with oil and grease contents generally less than 5 ppm. Rejection of chemical oxygen demand (COD) and biological oxygen demand (BOD5) were found to be 65% for UF treatment. In this work, Hermia's models were used to investigate the fouling mechanism involved in UF of the oily wastewater. The results showed that the best fit to experimental data corresponds to the cake layer formation model followed by the intermediate blocking model for both the UF membranes. For further treatment of the UF permeates, RO was applied using a thin film composite polyamide membrane. The rejection of COD, BOD5 and total dissolved solid (TDS) after UF/RO treatment increased up to 98%, 98% and 95%, respectively. The results showed that the final permeate has very high quality and even better than that is currently introduced to the cooling towers in Tehran refinery.

Permeate flux decline during UF of oily wastewater: Experimental and modeling

Oily wastewaters in Tehran refinery has become a big issue of water and environment pollution to be solved urgently. The results of an experimental study on separation of oil from oily wastewaters are presented. A polyacrylonitrile (PAN, 20kDa) membrane as the ultrafiltration (UF) membrane and an outlet wastewater of API unit of Tehran refinery were employed and the interaction effects of different operating parameters such as transmembrane pressure (TMP), cross-flow velocity (CFV), temperature and pH on permeate flux and fouling resistance were investigated. In this research, Taguchi method (L9 orthogonal array (OA)) was used initially to plan a minimum number of experiments.In this work, Hermia's models were used to investigate the fouling mechanisms involved in UF of oily wastewaters. The results showed that the best fit to experimental data corresponds to the cake layer formation model followed by the intermediate blocking model for all the experimental conditions tested. A more detailed analysis of the fouling mechanisms was studied via dividing the filtration curves into different regions corresponding to the different fouling mechanisms. Intermediate blocking and cake layer formation or combinations of them were found to occur during the UF experiments depending on the operating conditions.

Wellsite, Laboratory, and Mathematical Techniques for Determining Sorbed Gas Content of Coals and Gas Shales Utilizing Well Cuttings

Drill cuttings can be used for desorption analyses but with more uncertainty than desorption analyses done with cores. Drill cuttings are not recommended to take the place of core, but in some circumstances, desorption work with cuttings can provide a timely and economic supplement to that of cores. The mixed lithologic nature of drill cuttings is primarily the source of uncertainty in their analysis for gas content, for it is unclear how to apportion the gas generated from both the coal and the dark-colored shale that is mixed in usually with the coal. In the Western Interior Basin Coal Basin in eastern Kansas (Pennsylvanian-age coals), dark-colored shales with normal (∼100 API units) gamma-ray levels seem to give off minimal amounts of gas on the order of less than five standard cubic feet per ton (scf/ton). In some cuttings analyses this rule of thumb for gas content of the shale is adequate for inferring the gas content of coals, but shales with high-gamma-ray values (>150 API units) may yield several times this amount of gas. The uncertainty in desorption analysis of drill cuttings can be depicted graphically on a diagram identified as a “lithologic component sensitivity analysis diagram.” Comparison of cuttings desorption results from nearby wells on this diagram, can sometimes yield an unique solution for the gas content of both a dark shale and coal mixed in a cuttings sample. A mathematical solution, based on equating the dry, ash-free gas-contents of the admixed coal and dark-colored shale, also yields results that are correlative to data from nearby cores.

A New Zealand climatic template back to c. 3.9 Ma: ODP Site 1119, Canterbury Bight, south‐west Pacific Ocean, and its relationship to onland successions

Ocean Drilling Program Site 1119 is located east of South Island, New Zealand, in 394 m water depth on the upper continental slope. Though 93 km offshore today, during recent glaciations the site lay near the lowstand shoreline, directly beneath the path of seasonal riverine meltwater plumes. The core comprises an upper portion (0–86.19 metres composite depth (mcd)) deposited as upper slope clinoforms and a lower portion (to 513.5 mcd) deposited as mid‐slope, intermediate water depth sediment drifts, the two intervals being separated across a short c. 25 k.y. long unconformity within Marine Isotope Stage (MIS) 8. Almost throughout, the core comprises alternating micaceous muds and silts (glacials) and muddy, sometimes calcareous, sands (interglacials). Because of the enhanced potassium content of the terrigenous muds, the lithologies that characterise the two different climatic states possess markedly different natural gamma radiation signals, high and low, respectively. The natural gamma profile of Site 1119 therefore provides a high quality climatic time series with a 1–2 k.y. resolution (as sampled) back to the mid‐Pliocene. The cyclic gamma ray pattern provides a proxy measure of ice volume in the Southern Alps, and is therefore an atmospheric record. This record matches closely that of oceanic oxygen isotope curves back to MIS 100 (2.4 Ma) and less closely beyond to MIS Gi‐11 (3.91 Ma) at the base of the core. Marked, high gamma ray intervals at 3.68–3.63, 3.38, 3.12, and 2.80–2.67 Ma may reflect sharp mid‐latitude atmospheric coolings at these times, as supported by marine faunal and isotopic evidence elsewhere in the New Zealand region. Alternatively, they may in part reflect changes in clay provenance consequent upon tectonic uplift in the hinterland. The natural gamma measurements are consistent with an overall decline of 6°C in average atmospheric temperature over South Island, New Zealand, since 2.45 Ma. Milankovitch 41 k.y. cyclicity is also prominent in the natural gamma record over this period, and its 60 API unit magnitude implies temperature swings up to 12°C between glacials and interglacials (G/I). Similarly large G/I temperature changes occur also in nearby sea surface temperatures east of South Island and in atmospheric temperatures in Antarctica (Vostok), suggesting that Pleistocene climate signals were closely synchronised across wide areas of Southern Hemisphere middle and high latitudes. Together with changing tectonic and related palaeogeographic patterns, these temperature changes must have played a significant role in influencing the local time ranges of taxa used in biostratigra‐phy. Very few, if any, traditional New Zealand Pliocene‐Pleistocene index fossils have synchronous ranges across the region, and other methods of correlation (magnetostratig‐raphy, cyclostratigraphy, tephrochronology, isotope stratigraphy, and numeric dating) have therefore come to be of particular importance. The Site 1119 natural gamma record provides a unique reference template for climate variation in New Zealand mid‐latitudes, with which are compared the available correlation markers for both onshore and offshore Pliocene‐Pleistocene strata in the New Zealand region.

Recognition and Delineation of Paleokarst Zones by the Use of Wireline Logs in the Bitumen-Saturated Upper Devonian Grosmont Formation of Northeastern Alberta, Canada

The Upper Devonian Grosmont Formation in northeastern Alberta, Canada, is a shallow-marine carbonate platform complex that was subaerially exposed for hundreds of millions of years between the Mississippian(?) and Cretaceous. During this lengthy exposure period, an extensive karst system developed that is characterized by an irregular erosional surface, meter-size (several feet) dissolution cavities, collapse breccias, sinkholes, paleosols, and fractures. The karsted Grosmont Formation, which contains giant reserves of bitumen, subcrops beneath Cretaceous clastic sediments of the giant Athabasca tar sands deposit. The paleokarst in the Grosmont Formation can be recognized on wireline logs in relatively nonargillaceous carbonate intervals (<30 API units on the gamma-ray log) as excursions of the caliper log, off-scale neutron-density porosity readings, and severe cycle skipping of the acoustic log. The paleokarst is more prevalent in the upper units of the Grosmont Formation, and the effects of karstification decrease toward stratigraphically older and deeper units. The paleokarst usually occurs within 35 m (115 ft) of the erosional surface. The reservoir properties of the Grosmont Formation (e.g., thickness, porosity, permeability, and seal effectiveness) are significantly influenced by karstification. Depending upon the location, karstification has either benefited or degraded the reservoir characteristics. Benefits include porosity values greater than 40% (up to 100% in caverns) and permeability values of 30,000 md in severely fractured intervals. Detrimental reservoir characteristics include erosion, porosity and permeability reduction, and seal ineffectiveness.

Factors contributing to high gamma-ray levels in Upper Jurassic sandstone reservoirs of the Claymore Oilfield, North Sea

Sandstones of the Claymore Sandstone Member, which comprise the main reservoir in the Claymore oilfield, have gamma-ray intensities of 70–90 API units and locally reach 150 API units. A combination of techniques, including petrography, scanning electron microscopy, autoradiography and fission-track radiography, has been used to identify the sources of radioactivity. The generally high background is considered to be due to potassium feldspar concentrations of up to 27% and this is enhanced by the presence of radioactive heavy minerals, particularly zircon and monazite. Contributions to radioactivity from organic matter, mica, clays and lithic grains are minor. A failure to recognize the source of radioactivity in these sandstones can result in the assignment of high V shale values and low porosity during log analysis and productive zones may be underestimated.

The Stratigraphy of Selected Mission Canyon Wireline Log Markers, U.S. Portion of the Williston Basin, North Dakota

The Mission Canyon Formation along the northeast flank of the US Williston basin has been informally subdivided into intervals (members) based on wireline log markers. Wireline log responses of the markers are produced by both lithologic changes and radioactive elements present within these thin stratigraphic intervals. The wireline markers were originally described as transgressive events. Detailed stratigraphic analyses of the Sherwood and State A markers indicate they were deposited during progradation and sea level stillstand. A typical facies tract from east to west within the Sherwood marker contains anhydrites and anhydritic dolomites deposited in sabkha environments; patterned dolomudstones along shoreline trends (the Sherwood argillaceous marker); and limestones in shoaling environments along the Mission Canyon shelf (Sherwood gamma marker). During stillstand, brines produced in sabkha environments (east of the Sherwood shoreline) were enriched in magnesium and potassium. These brines migrated basinward first, dolomitizing mudstones. These brines were magnesium depleted by the time they reached shoals along the shelf. Potassium, however, remained in the system and is present within the marker along the shelf, as shown by a slight increase in API units on Spectrologs.

Recognition and Implications of Volcanic Glass Detritus in the Fluvial Deposits of the Middle Frio Formation, South Texas

ABSTRACT Detrital volcanic glass has been identified in fluvial deposits of the middle Frio Formation of South Texas. Petrographic analysis of core samples from Seeligson and Stratton fields reveals abundant unabraded to slightly abraded, very fine sand- to silt-sized glass shards. Gamma-ray log response correlates with the presence of volcanic glass. Volcanic glass and API unit counts markedly increase through the medial third of the middle Frio and diminish toward the upper third of the middle Frio. The stratigraphic and aerial distribution of volcanic glass detritus in middle Frio sandstones and mudstones suggests that volcanic ash was deposited rapidly on the Oligocene coastal plain of South Texas. Ash was apparently also deposited in the catchment basin of fluvial systems delivering sand to the Stratton area. Given the regional biostratigraphic constraints of middle Frio fluvial deposits, the interval containing volcanic glass is regarded as representing the 30- to 34-mya eruptive event in the Trans-Pecos region.

Oriskany Sandstone Lithofacies, Paleoenvironment, and Fracture Porosity in Somerset County, Pennsylvania: ABSTRACT

The Lower Devonian Oriskany (Ridgeley) Sandstone is an important deep exploration target in the Appalachian basin. In outcrop, the Oriskany is typically a calcareous quartzarenite with few discernible lithologic variations. Petrographic examination of two drill cores of the Oriskany-Needmore Shale interval from south-central Somerset County indicates that identifiable lithofacies of the upper Oriskany Sandstone exist in the subsurface. Oriskany lithofacies are defined as silica-cemented quartzarenite, sandy biosparite, calcareous-cemented quartz wacke, and coquinoid calcareous-cemented quartzarenite. These lithofacies are interpreted as depositional features of a shallow marine sand-bar complex, corresponding to central-bar, bar-margin, interbar, and storm-generated sheet sand (tempestite) depositional units. Paleoenvironmental interpretations are supported by diagnostic trace-fossil assemblages and coarsening-upward grain size trends indicative of vertically stacked marine bars. Correlation of gamma-ray logs from Oriskany wells across Somerset County suggests that these marine bars are laterally discontinuous and may change abruptly in thickness over relatively short distances. The basinal Needmore Shale overlies the Oriskany in this area, indicating a deepening of the depositional setting after Oriskany deposition. Although vertically oriented fractures were observed in all the Oriskany lithofacies and Needmore Shale, most of the fractures are healed by secondary calcite. Fracture porosity occurs primarily in the silica-cemented quartzarenite lithofacies, or central-bar paleoenvironmental unit. The quartzarenite lithofacies is recognizable on gamma-ray logs by its blocky, low API unit signature at the top of sequences that exhibit a downward increase in shale content. As the presence of fracture porosity is important in Oriskany natural gas production in this region, central-bar units are primary targets for exploration. End_of_Article - Last_Page 1930------------