Drilling Targets Research Articles

A web-based utility to highlight the role of remanent magnetization in Australian magnetic field data

Magnetic field data forms the most detailed and comprehensive geophysical coverage of Australia but at present there are challenges in reliable interpretation of that data. One major concern is the recognition and correct treatment of remanent magnetization which can result in mislocation of drill targets. To both reduce hazards of incorrectly interpreting magnetic field data and also to derive new geological information we have developed a web-delivered database of Australian magnetic anomalies recognised as being at least in part due to remanent magnetization. The database is linked with databases of model studies, magnetic moment analyses (MMA) and palaeomagnetic and rock magnetic investigations. These linked databases presented in the context of magnetic field imagery provide geoscientists with a rapid means of recognising the expression of remanent magnetization and of accessing information about that magnetization to assist in their interpretations. The databases have been lightly populated and can be accessed with a web utility. Community involvement is sought to further populate the databases with information currently distributed through geological survey, company and academic records.

Petroleum potential of the interpreted Paleozoic geoseismic sequences in the South Diyur Block, Western Desert of Egypt

ABSTRACT The South Diyur exploration block of nearly 38,000 sq km is located in the Farafra Oasis region in the Western Desert of Egypt. It is a frontier exploration area, the nearest well being Ammonite-1, a dry hole drilled by Conoco in 1979 immediately outside the southwestern corner of the block. The South Diyur Block is located on the probable northeast extension of the Kufra Basin in southeast Libya. Although prolific reserves of oil and gas occur in Paleozoic basins in North Africa and throughout the Middle East, to date, the targets for petroleum exploration in the northern Western Desert have been in Jurassic and Cretaceous rocks. The regional structural surface features in the South Diyur Block are the NE-trending Bahariya and Farafra anticlines interpreted as a deeply eroded and inverted Late Cretaceous structure on the southern extension of the Syrian Arc system. The oldest exposed rocks are a Cretaceous sequence of sublittoral sediments (the Campanian Wadi Hennis Formation) in the core of the anticline. The interpretation of the subsurface is based on 1,175 line-km of reprocessed 1970s-vintage 2-D seismic. Four sequence boundaries have been identified from the seismic data. SB-1 correlates with the Jurassic/Cretaceous boundary in Ammonite-1. SB-2 is regionally correlated with the Late Devonian to Early Carboniferous Hercynian unconformity that overlies deeply eroded and truncated Paleozoic sequences and possibly marks the regionally extensive Late Paleozoic basin inversion. SB-3 near the base of the interpreted Silurian sequence coincides with the ‘hot shale’ petroleum source rock that is present throughout North Africa and the Middle East. SB-4 is interpreted as a major unconformity at the top of an Upper Proterozoic sedimentary section that was misinterpreted as the Precambrian acoustic basement in Ammonite-1. Five seismic sequences relate to the seismic boundaries. SS-1, from the surface to SB-1 is characterized by subparallel seismic stratification and is composed mainly of sandstone with shale interbeds in Ammonite-1. SS-2, bounded by SB-1 and SB-2, is distinguished by parallel to subparallel seismic stratification. In Ammonite-1, the sequence of interbedded sandstone and shale is fresh-water bearing and lacking in top seals, thus reducing its prospectivity. The underlying SS-3 (SB-2 to SB-3) directly underlies the Hercynian unconformity and is characterized by semi-transparent seismic facies that may correspond to a thick Silurian shale sequence. SS-4 (SB-3 to SB-4) of probable Cambrian–Ordovician age has parallel seismic stratification. Deep channels are interpreted as evidence of a Late Ordovician–Early Silurian glacial phase that is present throughout North Africa and the Middle East. SS-5 (below SB-4) is marked by partial subparallel seismic stratification and block faulting. It probably belongs to the Late Proterozoic (Pan-African) phase of block faulting and pull-apart basins. Similar seismic geometries and facies occur in the Kufra Basin in southeast Libya and in many parts of the Arabian Plate, including the prolific petroleum systems of Oman. Exploration plays in the South Diyur Block are a combination of Paleozoic structural and stratigraphic traps associated with prospective fairways, and possible stratigraphic traps in the Late Ordovician–Early Silurian glacial channels. In addition, the interpreted Late Proterozoic sequences (SS-5) warrant further evaluation. In order to identify future exploration plays and drill targets, additional 2-D seismic (4,490 line-km), aeromagnetic and airborne gravity surveys will be integrated with the present seismic data and drilling results from Ammonite-1. This will allow a proper assessment of the magnetic basement, basin configuration and prospective fairways.

The application and value of AVO and spectral decomposition for derisking Palaeogene prospects in the UK North Sea

Perhaps over a billion barrels of oil, distributed over dozens of mostly small prospects, remain trapped in Palaeogene reservoirs in the UK sector of the North Sea Basin. Here we consider whether traditional amplitude versus offset (AVO) analysis, augmented by spectral decomposition, can provide a useful tool in helping to evaluate which of these prospects are worthwhile targets for drilling. In recent years, exploration drilling in the Palaeogene play fairway of the UK Northern North Sea has had mixed results. Eighty percent of wells drilled since 2000 have failed to encounter hydrocarbons, for reasons that include poor interpretation and evaluation of prospects, lack of AVO analysis, and positioning of wells on poor quality seismic data. Our analysis suggests that AVO analysis and spectral decomposition can be very useful and powerful tools for predicting the presence or absence of hydrocarbons in undrilled Palaeogene prospects in the North Sea. However, the validity of the results depends upon the availability of good quality seismic data with broad frequency content that have been acquired with cable lengths greater than 3 km.

A Technical and Economic Study of Completion Techniques in Five Emerging US Gas Shales: A Woodford Shale Example

SummaryWith the increased demand for energy and the declining conventional hydrocarbons worldwide, energy companies are turning to unconventional resources such as shale gas. With more than 2,000 Tcf of gas in place indentified in just 5 shale gas plays in the United States, shale-gas formations are now the number one targets for exploration drilling. Furthermore, there are still many more major shale-gas plays and basins waiting to be explored, evaluated, and developed. Because of the extremely low permeability of most shale formations, it is essential to select the appropriate completion techniques for shale-gas reservoirs. There are very few papers in the petroleum literature that provide a logical method to select completion techniques for given shale-gas-reservoir conditions. There are papers discussing successful completion techniques that seem to work for a specific shale. We have used many of these SPE papers to help define "best practices" in completing shale-gas reservoirs. We then developed logic to determine the best practice in completing shale-gas reservoirs as a function of reservoir conditions. In this paper, we will specifically cover the logic we have developed for choosing completion techniques in shale-gas reservoirs.First, we performed a literature review on the five basins as well as on all shale-gas plays in the US to determine the best practices in shale-gas completion techniques in fluctuating price environments and identify key geologic parameters that affect overall well performance. From our literature review, we identified seven pertinent geologic parameters that influence shale-gas completion practices. Next, we identified different completion trends in the industry for different geologic settings. Subsequently, we generated an economic model and performed sensitivity analysis to determine optimal completions for each gas-shale basin. On the basis of these economic models, we developed decision flow charts to select completion techniques. Finally, we programmed the flow chart, and we call this program Shale Gas Advisor. This program can be used to determine optimum completion best practices not only for the five gas-shale basins discussed, but also for gas-shale plays that have similar geologic attributes. We validated the program with published case histories in the SPE literature.

Origin and architecture of fractured-cavernous carbonate reservoirs and their influences on seismic amplitudes

Ordovician carbonate is important for petroleum exploration and production in the Tarim Basin. The matrix porosity of the carbonate reservoirs is usually less than 2% because of their age-old sedimentary history and depth (deeper than 5500 m). The storage spaces are secondary dissolution pores and fractures which are dominated by visible dissolution caves, holes, and fractures (Zhou et al., 2006). The reservoirs are therefore called fractured-cavernous reservoirs. Good quality reservoir usually appears as a strong reflection (called “bead-like reflection” or “BR”) on poststack seismic sections, and the corresponding reservoir is called a fracture-cave body (FCB) which is the main drilling target of current E&P in Tarim Basin. The FCBs are unevenly distributed in carbonate strata, and the storage spaces inside an FCB are also unevenly distributed. Therefore, the reservoirs have both macroscopic and microscopic heterogeneities, which bring great challenges to petroleum exploration.

The application of remote sensing and structural analysis in groundwater exploration in basement terrains, Darfur region, Western Sudan

Darfur region is one of the most vulnerable areas in Sudan that suffer from shortage in water supply. The objective of the current study is to utilize remote sensing techniques combined with the structural analysis to recognize the most potential fracture zones for groundwater occurrences in the hard rock terrains of Darfur region. The old ductile deformation features in Darfur region delineated from Landsat imageries are used for the structural analysis to determine and classify the fractures in the hard rock terrains of the region. Based on the structural analysis conducted in this study, Darfur region was divided into two domains—the western domain of the pre-Pan-African age that is affected by the two deformational phases (D1 and D2), and the eastern domain represented by the basement related to the Pan-African orogeny. The most potential fractures in the western domain are in the NW–SE and NE–SW trends that classified as extensional and release open fractures for the deformations D1 and D2. In the eastern domain, the main potential fractures for groundwater occurrence are in E–W and N–S directions that are classified as extensional and release open fractures of the deformation D3. From the results of the structural analysis, the main potential fracture systems in Darfur region trend are NW–SE, NE–SW, and E–W directions. The intersections of these fracture systems are the most promising targets for drilling, with consideration of the topography, the rates of recharge, and the underlying geology. The geophysical data and boreholes information in Zalingei and north of El Geneina areas in West Darfur confirm the results obtained from remote sensing data and structural analysis, in which the NW–SE, NE–SW, and E–W fractures trends are the most potential fractures in Darfur region.

Subsurface gas hydrates in the northern Gulf of Mexico

The northern Gulf of Mexico (GoM) has long been a focus area for the study of gas hydrates. Throughout the 1980s and 1990s, work focused on massive gas hydrates deposits that were found to form at and near the seafloor in association with hydrocarbon seeps. However, as global scientific and industrial interest in assessment of the drilling hazards and resource implications of gas hydrate accelerated, focus shifted to understanding the nature and abundance of “buried” gas hydrates. Through 2005, despite the drilling of more than 1200 oil and gas industry wells through the gas hydrate stability zone, published evidence of significant sub-seafloor gas hydrate in the GoM was lacking. A 2005 drilling program by the GoM Gas Hydrate Joint Industry Project (the JIP) provided an initial confirmation of the occurrence of gas hydrates below the GoM seafloor. In 2006, release of data from a 2003 industry well in Alaminos Canyon 818 provided initial documentation of gas hydrate occurrence at high concentrations in sand reservoirs in the GoM. From 2006 to 2008, the JIP facilitated the integration of geophysical and geological data to identify sites prospective for gas hydrate-bearing sands, culminating in the recommendation of numerous drilling targets within four sites spanning a range of typical deepwater settings. Concurrent with, but independent of, the JIP prospecting effort, the Bureau of Ocean Energy Management (BOEM) conducted a preliminary assessment of the GoM gas hydrate petroleum system, resulting in an estimate of 607 trillion cubic meters (21,444 trillion cubic feet) gas-in-place of which roughly one-third occurs at expected high concentrations in sand reservoirs. In 2009, the JIP drilled seven wells at three sites, discovering gas hydrate at high saturation in sand reservoirs in four wells and suspected gas hydrate at low to moderate saturations in two other wells. These results provide an initial confirmation of the complex nature and occurrence of gas hydrate-bearing sands in the GoM, the efficacy of the integrated geological/geophysical prospecting approach used to identify the JIP drilling sites, and the relevance of the 2008 BOEM assessment.

Geochemical Exploration for Gold Through Transported Alluvial Cover in Nevada: Examples from the Cortez Mine

Geochemical orientation surveys were completed over covered Carlin-type gold deposits in the Cortez mine area with the expressed aim of identifying and evaluating exploration methods to discover Au ore under transported alluvial cover in Nevada. Orientation tests were designed to assess the utility of geochemical applications at various scales of exploration, both drill targets at the deposit scale and “footprints” associated with deposits at the district scale. Detailed surveys were completed over the covered Gap deposit, located adjacent to the giant Pipeline deposit. Both Carlin-type gold mineralization and earlier, spatially associated, base metal skarn mineralization at Gap were located by soils, soil gas, and vegetation. Loam soils at 6- to 12-cm depth provided a consistent and uniformly available sample medium. Gold ore under 10 m of cover in the northern portion of the Gap deposit was readily detected by analysis of Au by fire assay and ultra trace aqua regia methods in the −80 mesh fraction of these loam soils. Arsenic anomalies occur over the northern end and over the main ore zone at Gap, where there is 25 to 50 m of alluvial cover. Zinc concentrations in soil show the most coherent spatial relationship with underlying Au ore. Tests of MMI-B and Enzyme Leach selective leaches did not result in significant enhancement of the anomalies relative to aqua regia. CO2 and O2 in soil gas indicate faults and underlying mineralized carbonates at Gap, where weathering reactions likely generated CO2 from acid reaction with carbonate. Elevated Au and As in mixed sagebrush and shadscale occur over most ore zones, with the highest Au concentrations over the main ore zone rather than the shallowly buried northern zone. Like soils, elevated Zn in vegetation shows the most coherent spatial relationship with underlying ore. Assays from 1,859 drill holes show a 4- to 5-km2 “footprint” at the basin gravel-bedrock unconformity centered on the Pipeline Carlin-type gold deposit, where samples of basal alluvium provide a large, coherent >50 ppb Au anomaly. Enriched As, Tl, K, and F in alkaline groundwater sampled from monitoring wells surrounding the Pipeline open pit provide a ≥5-km2 hydrogeochemical footprint. Higher As and Tl concentrations occur down gradient from Pipeline indicating Carlin-type Au mineralization is the source of the enrichment. The surface metal anomalies are consistent with upward migration of metals through fractured alluvial cover. Likely metal transport mechanisms include barometric pumping of gases or seismic pumping of groundwater. Upward diffusion of metals through the thick vadose zones in northern Nevada is not a viable process. Surface anomalies over Gap and other deposits appear to be mature and may have developed over millions of years. In such mature anomalies, much of the metal that migrated from underlying bedrock is probably hosted in resistant secondary minerals, which are more readily dissolved by aqua regia than by various selective leaches. Sampling of soil gas and soils is appropriate at the drill target scale. Vegetation should be sampled when consistent soils are not available. Sample spacing should be adjusted according to “real-time” soil gas readings in order to increase sampling density over fracture zones. The patterns of >50 ppb Au for both the top of bedrock and base of alluvium at Pipeline provide useful templates for comparison with other drilling programs through cover. Groundwater is an effective and under-utilized reconnaissance-scale sample medium. Gold is likely to be soluble in neutral to alkaline groundwater in Nevada, and could provide a direct indicator of blind covered ore.

Discoveries of Low-Sulfidation Epithermal Au-Ag Veins at Cerro Negro, Deseado Massif, Argentina

ABSTRACT Exploration by Andean Resources Ltd. in the Cerro Negro district of southern Argentina between March 2005 and July 2010 delineated resources of 2.54 Moz gold and 23.5 Moz silver in three low-sulfidation epithermal vein deposits. Two of these, Eureka West and Bajo Negro, are new discoveries; the third deposit, Vein Zone, had been explored previously. Additional measured and indicated resources of 2.7 Moz Au have been estimated by current owners Goldcorp Inc. for three other discoveries at Cerro Negro: San Marcos, Mariana Norte, and Mariana Central. The total gold resource for the project is 5.3 Moz Au measured and indicated, plus 1.24 Moz inferred. Andean completed a positive feasibility study into an underground and open pit mining operation in July 2010, based on reserves of 2.07 Moz Au and 20.6 Moz Ag at Eureka, Bajo Negro, and Vein Zone. The study concluded that these deposits can produce an average of 200,000 oz of gold and 1.8 Moz of silver per year for 10 years, which would make Cerro Negro the largest gold producer in the Deseado Massif without considering resources or production from the other three deposits. The deposits are hosted by Late Jurassic volcanic and probable intrusive rocks of the Deseado Massif in which Triassic-Cretaceous extension created the structures which control the precious metal vein deposits. Four of the five new discoveries were found through geologic mapping, but pioneering work by previous explorers also contributed; the two most recent discoveries do not crop out. Geophysical studies helped to define drill targets and are likely to be increasingly important in future exploration. A key ingredient in Andean’s success was a willingness to drill test targets identified by a skilled and dedicated, largely Argentine, exploration team, supported by Australian and North American-based management.

Innovative data inference

The goal of exploration geophysics is to infer the nature of buried structure and, in particular, generate drill targets that lead to a mineral deposit discovery or reserve delineation. As a profession, we aim to turn geophysical data into geological information. Most geophysical techniques enable inferences to be made from airborne, ground-based or bore-hole data through a deterministic process whereby a single model ‘answer’ is generated. Well-founded algorithms include uncertainty estimates for different parameters in the model and/or some form of model validation. This approach has been successful to date, and we advocate the continued use of deterministic algorithms.We also advocate that alternate strategies for extracting information from data are used alongside deterministic strategies. If we consider two general properties of data inference approaches, that of (1) assurance and (2) opportunity, deterministic approaches score poorly regarding opportunity: that is, useful answers may be missed. Alternate strategies can be computationally intensive, but several important classes of approach, summarised in this article are now tractable on workstation or high-specification notebook PCs. By using a range of strategies we can maximise both assurance and opportunity for a particular data inference goal and obtain extra, useful geological information from our data.

Assessment of Quality of Assay Data on Drill Samples from Golden Star Resources (Prestea/Bogoso)Ltd

The success of a mining company is dependent on the integrity of the resource database. The quality of assay data and thus the validity of the database can only be guaranteed when appropriate sampling and assaying protocols have been implemented. It is necessary to convince investors and project financiers that appropriate samples have been collected and that analyses are valid. Monitoring laboratory data improves confidence in ore reserves, drill target selection and management of the company. This work examines how the key sampling, analytical and quality assurance factors impact on project success and assaying samples from Golden Star Resources (Prestea/Bogoso) Limited. It describes Fire assay and Screen Fire Assay, a ‘high tech’ method for gold analysis. The study has shown that Screen Fire Assay is a more suitable and reliable analytical method for ores with coarse gold particles than fire assay. It is therefore recommended that the Screen Fire Assay method be used as a model for gold exploration in general and for ores with coarse gold problems in particular.

Exploration of the Yandal gold province, Yilgarn Craton, Western Australia

The opening of the Yandal gold province during the 1990s provides a detailed case history of repeated exploration successes. First highlighted in 1992 with the discovery of the Bronzewing gold deposit, the resources of the Yandal gold province grew rapidly through the 1990s with a series of discoveries. By 2000, the gold endowment of the province was 470 tonnes (t), and it was producing 25 t Au p.a. from the Jundee, Bronzewing and Darlot mines. The exploration activities in the deeply weathered and mostly covered Archaean Yandal greenstone belt during this period set the standard for Australian gold exploration in deeply weathered terrains through a twofold approach that involved drilling 100 000 holes, and at the same time making a strong commitment to ore deposit research. Confidence came from an understanding of Archaean gold deposits and the regolith, including the effects of gold dispersion in the weathered zone and the necessity to properly penetrate the Cainozoic cover for effective drill testing. Special emphasis was placed on gold distribution, alteration, lithologies and structural geology, and these soon became the focus of both data collection and drill targeting during the exploration process. Aeromagnetic data provided the platform for interpolation between geologically constrained areas in this province of less than 10% outcrop. Rapid and effective communications and data handling were critical issues for managing large exploration programs involving many geologists in remote areas.

The Pleistocene Cooling Built Challenger Mound, a Deep-water Coral Mound in the NE Atlantic: Synthesis from IODP Expedition 307

IODP Expedition 307 revealed the interior of a deep-water coral mound in NE Atlantic for the first time and improved our understanding of these intriguing structures. From the summit of our drilling target, Challenger Mound at ~800 m deep in the Porcupine Seabight, south west of Ireland, we recovered the entire mound section of 155 m long, which almost entirely consists of coral-bearing sediments and rests on the Miocene siliciclastics. The mound initiation is temporally correlated to the global cooling at the beginning of Pleistocene, when modern circulation was established in Atlantic.A key oceanographic feature of the mound provinces is the density gradient that developed above the saline Mediterranean Outflow Water where organic particles persist for a longer time and fuel the coral communities. Growth of the deep-water mounds reflected the glacial/interglacial change. Our age model recognized two growth stages separated by a substantial hiatus; the depositionally continuous lower mound (2.61.7 Ma) accumulated under the low-amplitude relative sea-level change, and the discontinuous upper mound (1.0 Ma to midHolocene) developed under the high-amplitude relative sea-level change. Low cellular abundances in the geochemical features of the mound sediments did not support the hypothesis that hydrocarbon seepage and associated microbial activity significantly enhanced mound initiation and development.

Low-temperature geothermal utilization in Iceland – Decades of experience

Geothermal energy plays a key role in the economy of Iceland and it supplies about 89% of the space heating requirements. A large fraction of the country's district heating services (hitaveitas) use energy from low-temperature geothermal systems, which are mostly located outside the volcanic zone. Many of the geothermal district heating services have been in operation for several decades and much can be learned from their operation, in particular regarding long-term management of low-temperature geothermal resources. In most cases down-hole pumps are used, but there are examples of large-scale artesian flow still being maintained. The Reykjavík geothermal district heating service is the world's largest such service. It started operation on a small scale in 1930, and today it serves Reykjavík and surrounding communities, about 58% of the total population of Iceland. The Reykjavík district heating service utilizes three low-temperature systems. The production and response (pressure, chemistry, and temperature) histories of these systems and six other low-temperature geothermal systems are discussed. Four of the systems are very productive and reach equilibrium at constant production. Two are much less productive and do not attain equilibrium, while three are of intermediate productivity. Groundwater inflow has caused temperature decline and chemical changes in two of the systems. Several problems have faced the Icelandic low-temperature operations, such as excessive pressure drawdown caused by overexploitation, colder water inflow, and sea water incursion. None of the district heating systems has ceased operation and solutions have been found to these problems. The solutions include improving the energy efficiency of the associated heating systems, deeper and more focussed drilling (e.g., directional drilling), finding new drilling targets (even new drilling areas), and injection, as well as technical solutions on the surface. The long utilization case histories provide important information pertaining to sustainable management of geothermal resources.

Geochemical exploration in Cornwall and Devon: a review

ABSTRACT Cornwall and Devon host some of the most significant metallic mineralization in the UK and were an obvious location for geochemical research. Professor John Stuart Webb (1920–2007) and his Applied Geochemistry Research Group at Imperial College, London, provided a framework for much exploration through the ‘Wolfson Geochemical Atlas of England and Wales’, as well as a number of related projects. The regional geochemistry was followed-up by a number of commercial and government surveys, mainly using residual soils. These have validated many of the techniques developed at Imperial College although no new mines have resulted. Surveys around the Wheal Jane mine and north of the St. Austell granite have located new areas of mineralization associated with the main phase of vein formation, as well as tourmaline-rich breccias and calc-silicates. The distribution of pathfinder elements for Sn and W deposits is erratic and As enrichment appears to be related to distinct host sediments and granite phases. Base metal exploration has defined areas of enrichment within lower Carboniferous sediments, which were detected in the early Imperial College surveys. Deep overburden sampling defined drill targets which intersected sub-economic mineralization. Gold exploration defined a number of targets which appeared largely to be the result of a number of phases of heavy mineral concentration. An exception was low-grade mineralization in Permian volcanic rocks. Very little work has been done on the subsurface signature of the deposits and their relation to the differing granites. This, which was partly the focus of Webb9s early work, is a priority for further study.

Postglacial Faults in Fennoscandia: Targets for scientific drilling

During the last stages of the Weichselian glaciation (ca. 9,000–15,000 years B.P.), reduced ice loads and glacially affected stress fields resulted in active faulting in Fennoscandia with fault scarps up to 160 km long and up to 30 m high. These postglacial (PG) faults are usually SE dipping, SW–NE oriented thrusts, and represent reactivated, pre-existing crustal discontinuities. Postglacial faulting indicates that the glacio-isostatic compensation is not only a gradual viscoelastic phenomenon, but also includes unexpected violent earthquakes, suggestively larger than other known earthquakes in stable continental regions. We explore here possibilities and benefits for investigating, via scientific drilling, the characteristics of postglacial faults in northern Fennoscandia, including their structure and rock properties, present and past seismic activity and state of stress, as well as hydrogeology and associated deep biosphere. The research is anticipated to advance science in neotectonics, hydrogeology and deep biosphere studies, and provide important information for nuclear waste disposal, petroleum exploration on the Norwegian continental shelf and studies of mineral resources in PG fault areas.

Integrated approach to geomodelling and dynamic simulation in a complex mixed siliciclastic–carbonate reservoir, N'Kossa field, Offshore Congo

Abstract The Lower Sendji Carbonate (Albian age) of the N'Kossa field is a mixed siliciclastic–carbonate reservoir exhibiting very heterogeneous reservoir properties and the development of extensive vertical flow permeability barriers. The reservoir is a succession of interstratified dolomite, limestone, sandstone and shale lithologies of sabkha, tidal flat and lagoonal origin. Early, synsedimentary dolomite cements are extensively developed, particularly over local palaeohighs. Challenges to hydrocarbon production include: (1) the vertical variability of reservoir properties; (2) the presence of many, laterally extensive vertical flow barriers; and (3) variable connectivity between fault-bounded compartments. This complexity was underestimated at the appraisal stage, the initial development plan calling for a relatively simple scheme of pressure support to the critical reservoir fluid via injection into the gas cap and the water leg. This decision was supported by the identification of a single hydrocarbon column (>400 m thick) over the entire structure which suggested a lack of vertical compartmentalization. Recent studies, which include a geological synthesis of more than forty wells and a dynamic data synthesis to determine the production mechanism and identify key heterogeneities, show a lack of pressure support to the oil leg from both water and gas injector wells. Today, integration of static and dynamic data together with an improved geological understanding of the stratigraphic control on vertical flow barriers and mapping of high permeability sandstone layers is the key to identifying unswept zones of the field and future infill drilling targets.

Utility of the 2-D Multi-Electrode Resistivity Imaging Technique in Groundwater Exploration in the Voltaian Sedimentary Basin, Northern Ghana

Guinea worm and other water borne diseases associated with untreated surface water use have necessitated that communities are supplied with groundwater in the Northern Region of Ghana. The Electrical Resistivity Survey (ERS) method is one of the common geophysical survey techniques used for borehole siting in Ghana. Various groundwater exploration programs have utilized it by employing the four electrode system. However, the ERS has not been able to locate potential borehole sites successfully in the Voltaian Sedimentary Basin (VSB) in the Northern Region of Ghana although it has been successful in other geological formations. Unsatisfactory results obtained from the employment of the ERS system in the VSB therefore necessitated an experiment with the 2-D Multi-Electrode Resistivity Imaging (2-D MERI) technique. Field results show that the 2-D MERI is a robust and efficient technique: an improvement on the four electrode ERS system in terms of amount and quality of data obtained. The 2-D MERI interpretations were confirmed with a much improved borehole drilling success rate of 60% compared to 38% obtained using the ERS within the mudstone and shale formations. The 2-D MERI also indicated that very low resistivity layers in the VSB may not necessarily be good targets for borehole drilling and the generally held notion that groundwater cannot be found at depths more than 25 m is no longer tenable.

Geophysical signatures of some recently discovered large (> 40 ha) kimberlite pipes on the Alto Cuilo concession in northeastern Angola

This paper presents a comparison of geophysical responses from several large kimberlite complexes discovered and delineated on the Alto Cuilo concession in the diamond fields of northeastern Angola in the years 2005 to 2008. Several geophysical methods were used in combination with geochemical and mineralogical prioritization techniques to guide exploratory, delineation and bulk sample drilling, in order to rapidly identify and evaluate the kimberlite bodies. The kimberlites were emplaced through Karoo Supergroup sandstones and shales, have eruption ages contemporaneous with the sand-dominated Cretaceous-age Calonda Formation, and are covered by sand-dominated poorly consolidated sediments of the Kalahari Group. Given that sand-dominated non-kimberlite lithologies are magnetically transparent, a low level, high resolution helicopter-borne magnetic gradiometer survey proved to be exceptionally effective in discriminating kimberlite targets, even for low-amplitude anomalies (e.g. 1–2 nT). The helicopter magnetic data outlined approximately 244 probable kimberlite targets and drilling of 103 targets confirmed 80 new kimberlites greater than 5 ha in area. Most kimberlites take the form of well-preserved crater edifices containing a full range of crater-related kimberlite lithologies. Ground gravity and electromagnetic surveys were conducted over all kimberlites prioritized for follow-up investigation. Geophysical responses were ground-truthed against magnetic susceptibility and density measurements, which were routinely collected on all drill cores. The geophysical signatures resolved by the three independent geophysical methods were surprisingly variable and are inferred to be sourced primarily in the crater facies materials, which demonstrate characteristically variable lithologies. Geophysical interpretations guided the drill targeting at all stages of the program at Alto Cuilo, from exploration to evaluation. Combined with geochemical and mineralogical prioritization techniques, the geophysical signatures in magnetic, gravity and electromagnetic data provide a sound basis to guide exploratory, delineation and mini bulk sample drilling of kimberlites buried under 10–70 m of overburden. The significant variability in the geophysical responses from kimberlites that are similar in size, structure and geometry highlights the importance of applying all three independent geophysical methods in order to effectively achieve kimberlite exploration and evaluation goals. The effectiveness of a multidisciplinary approach to kimberlite evaluation is demonstrated in the rapid assessment of a cluster of large kimberlites discovered at Project Alto Cuilo.

A case study of deep electromagnetic exploration in conductive cover

This paper describes the survey design, data collection and results from a geophysically driven exploration program in Northern Australia. The exploration project was a joint venture between Falcon Minerals NL and Anglo American. It was focussed on finding massive to net textured nickel sulphides associated with a mafic intrusive body. The extent of the intrusive body was inferred from interpretation of a significant 14mgal gravity anomaly. Limited previous exploration indicated any drill targets were likely to be covered by 400 to 500m of mesozoic and recent sediments. These sediments are estimated to have a conductance of 200Siemens, making it difficult to use conventional electrical geophysics for effective exploration. Anglo American, together with its research partner IPHT of Germany have developed a low temperature superconducting quantum interference device (LTS) that can be used as a sensor in transient electromagnetic (TEM) surveys. The LTS is a ?B field? sensor with significant advantages in noise levels over other ?B field? sensors and the ability to detect targets with time constants up to 3 orders of magnitude larger than conventional dB/dt coils (Le Roux et al 2007). The LTS sensor is particularly suited to finding highly conductive targets beneath conductive cover. After forward modelling and survey design, the JV completed some 270km of reconnaissance moving loop ground electromagnetic surveys. Selected anomalies identified with this survey were then followed up with more detailed moving loop and some fixed loop surveys. These results were then modelled using a variety of software packages to try and fit the late time TEM field data. Drilling of selected targets and some downhole TEM surveys are in progress.