Pore-filling Carbonate Cements Research Articles

Pore Structure and Fractal Characteristics of the Middle and Upper Permian Dalong and Gufeng Shale Reservoirs, Western Hubei Province, South China

The Middle and Upper Permian Dalong and Gufeng Formations in South China have recently been considered as potential gas-producing shales. However, their pore structure characteristics remain poorly understood. To investigate the pore structure and fractal characteristics of the pores in these two formations, a suite of shale samples from the Dalong and Gufeng Formations in the western Hubei Province, South China were analyzed by multiple techniques, namely, TOC content, X-ray diffraction (XRD) mineralogy analysis, optical microscopy observations, major elemental analysis, field emission-scanning electron microscopy (FE-SEM), and low-pressure gas adsorption measurements (N2 and CO2). The identified major shale lithofacies include siliceous mudstone, carbonaceous mudstone, argillaceous-siliceous mixed mudstone, and calcareous-siliceous mixed mudstone. SEM images show that the dominant pore types include the pores between brittle minerals, slit-shaped pores between clay sheets, and secondary organic matter (OM) pores within solid bitumen. The pore size distribution is dominated by micropores and mesopores (<30 nm), which are the major contributors to total pore volume and surface area for the Dalong and Gufeng Formations. Based on the Frenkel–Halsey–Hill (FHH) method, fractal dimensions (D1, D2) calculated from the nitrogen adsorption data have a range of from 2.489 to 2.772 (D1) and from 2.658 to 2.963 (D2), and are higher in the Gufeng Formation (average TOC = 8.3 wt.%) due to a higher TOC content comparing to the Dalong Formation (average TOC = 6.2 wt.%). The pore development and fractal characteristics are primarily controlled by organic matter (OM), carbonate minerals, and clay minerals for both the Dalong and Gufeng Formations. Shale samples with high TOC content, low carbonate content, and high clay content tend to develop more heterogeneous micropores and mesopores, which is ascribed to the generation of clay-related and OM-hosted pores, along with the destruction of primary pores by pore-filling carbonate cements.

Diagenesis as a control on the tight sandstone reservoir quality of the upper Carboniferous strata in the northeastern Ordos basin, China

The upper Carboniferous tight sandstone strata are considered the most promising targets for gas exploration in the northeastern Ordos Basin. Reservoir quality is critical for the successful commercial exploration and development of tight sandstone gas. Reservoir quality of deeply buried tight sandstones is controlled by diagenesis, yet there is sparse systematic documentation on upper Carboniferous tight sandstone reservoir quality and its diagenetic link. In this study, we utilized core analysis and wireline log data, and core samples were studied using X-ray diffraction, optical light microscopy, scanning electron microscopy, porosity–permeability, mercury intrusion capillary pressure and nuclear magnetic resonance measurements. Dolomite, siderite, quartz, kaolinite, and illite are the main diagenetic cements. The upper Carboniferous sandstones pore system consists of residual intergranular pores, intragranular dissolution pores associated with feldspar grains and micropores mainly related to diagenetic minerals. Due to differences in diagenetic alterations, sandstones have significant differences in pore types and pore-filling components, ultimately leading to differences in reservoir quality. The residual intergranular porosity is closely related to the reservoir quality. Quartz cement and authigenic clay minerals lead to reduced residual intergranular porosity, but their impact on reservoir quality is relatively limited due to their relatively low content. The most significant factors contributing to reservoir quality deterioration are ductile (clay-rich) grain-influenced compaction and pervasive pore-filling carbonate cement. When sandstone contains large numbers of ductile grains, it loses a large amount of intergranular porosity due to compaction, and the pore space contains only a small number of dissolution pores and micropores, resulting in reservoir quality deterioration. Similarly, when a large amount of carbonate cement is developed in sandstone, it leads to intergranular porosity loss. The best reservoir quality sandstones preserve a percentage of intergranular porosity because they (i) have few ductile grains, resulting in low compaction, and (ii) do not have significant amounts of carbonate cements. Finally, the diagenetic variations were successfully correlated with wireline logs to evaluate the reservoir quality of subsurface upper Carboniferous sandstones. The data sets provided in this research offer insights to better evaluate and predict reservoir quality in the northeastern Ordos Basin.

Diagenesis and petrophysics of Miocene sandstones within southern Apennines foreland, Italy

Deep-marine clastic turbidite beds commonly occur in the Southern Apennines and record the main tectonic phases associated with the structural evolution of the area. Based on variations of detrital compositions through time, the sandstone suites are traditionally subdivided into four petrofacies: (I) quartzolithic (Burdigalian-Langhian), (II) quartzofeldspatholithic (Langhian-Tortonian), (III) quartzofeldspathic (Serravallian-Tortonian), and (IV) arkose (Tortonian-Messinian) sandstones. Since the complexity and diversity of these sandstone petrofacies, we aim to investigate their major diagenetic events and relate them with their detrital modes and physical properties in order to obtain key basic information at a regional scale, to better understand the main controls on petrophysical properties in this and in similar contexts. The petrophysical analysis reveals that the quartzolithic sandstones have the lowest porosity values, with a median of 4.56%. In contrast, porosity increases to median values of 16.75% in the quartzofeldspathic and arkosic petrofacies. The median permeability values range between 4.85 and 236.38 mD, and its distribution is generally poorly correlated with any other petrophysical parameter. The most widespread diagenetic minerals observed are carbonates, clay minerals, Fe-oxides and hydroxides, and less quartz. These sandstones are characterized by high compaction, which is the primary factor controlling porosity loss since the early diagenetic phases. In contrast, the early precipitation of pore-filling carbonate cement is assumed to have partly reduced the effect of compaction in quartzolithic and arkosic sandstones. Nonetheless, variations in petrophysical features among petrofacies can be traced back to the detrital composition, with the highest but less effective porosity associated with the presence of ductile grains, siliciclastic matrix and feldspars. Accordingly, this study reveals how linking detrital modes to the key petrophysical parameters may help obtain significant information about the regional control on the evolution of porosity and permeability in clastic turbiditic wedges of foreland domain settings as a baseline useful to further local and detailed studies.

Petrology, Diagenesis and Reservoir Quality in the Hawkesbury Sandstone, Southern Sydney Basin, Australia

Petrography of the Hawkesbury Sandstone was described by thin section, scanning electron microscope and Xray diffraction techniques. Quartz is dominant in the Hawkesbury Sandstone which contains very minor feldspar, lithic grains, mica and heavy minerals. The Hawkesbury Sandstone is quartzarenite to sublitharenite. Quartz includes monocrystalline and polycrystalline grains while the feldspar includes both K-feldspar and plagioclase. Volcanic, sedimentary and chert rock fragments are present. Thin section porosity occurs in the Hawkesbury Sandstone, particularly the coarse-grained deposits. Primary porosity is more common than secondary porosity in the Hawkesbury Sandstone. Thin sections and scanning electron microscopy were used to describe diagenetic alterations and their influence on porosity in Hawkesbury Sandstone. These diagenetic alterations include compaction, quartz overgrowths, authigenicclay minerals and carbonate cement. Compaction occurred in the Hawkesbury Sandstoneduring throughout diagenesis. The influence of chemical compaction on thin section porosity was greater in the Hawkesbury Sandstone. Quartz overgrowths are common in the Hawkesbury Sandstone where they have a strong influence on porosity. Authigenic clay minerals are the widespread in the Hawkesbury Sandstone filling pores and occurring as grain-coatings on detrital and authigenic grains. Where they coat quartz grains they preserve porosity by preventing growth of quartz overgrowths. Pore-filling carbonate cement reduced porosity whereas dissolution of carbonate resulted in secondary porosity. Dissolution of unstable feldspar and lithic grains is absent in the Hawkesbury Sandstone. The Hawkesbury Sandstone is characterized by primary and secondary porosity, thus it has good groundwater storage and flow potential. Medium- and coarse-grained sandstone beds are common in the Hawkesbury Sandstone but it shows vertical variations in porosity. A few impermeable shale and siltstone units occur in the Hawkesbury Sandstone forming local confining layers.

Microbially mediated iron ore formation, Silicic Superunit, Rudabánya, Hungary

The mineralogical, petrological, geochemical and textural characterization of the Triassic Bódvaszilas Sandstone in the Rudabánya open pit and Aggtelek Mts. is summarized based on our investigations. The pore-filling carbonate cement in the Lower Triassic Bódvaszilas Sandstone Formation of a proximal basin series (Rudabánya) was formed as the result of syngenetic and early diagenetic two step microbially mediated Fe mineralization (Fe-oxide formation by chemo-lithoautotrophic Fe-oxidizing microbes (FeOB) in a suboxic neutrophylic environment, and Fe-carbonate by heterotrophic microbes in a suboxic to anaerobic environment based on mineralized microbially produced textural evidences). The carbonate in the higher parts of the formation (transition between the Bódvaszilas Sandstone and the overlying formation, Szin Marl) was formed by the same metasomatic processes that produced the siderite ore in the Middle Triassic limestones and dolomites. These processes led to partial or complete Fe replacement of the syngenetic carbonate layers. Microbially mediated Fe enrichment also occurred at distal basin position in the Bódvaszilas Sandstone (Aggtelek Mts.). The formation later underwent deep burial diagenesis.

Distribution of diagenetic alterations in fluvial channel and floodplain deposits in the Triassic Narrabeen Group, southern Sydney basin, Australia

In the Narrabeen Group, floodplain shale and siltstone are abundant in the Wombarra Claystone, Stanwell Park Claystone and Bald Hill Claystone whereas the Newport Formation is a fine-grained lagoonal deposit. Mediumand coarse-grained fluvial channel sandstone is common in the Coalcliff, Scarborough and Bulgo. Sandstone composition includes mostly lithic grains with minor quartz, feldspar, mica and heavy minerals; litharenites and sublitharenites are common and quartzarenites are rare. The diagenetic alterations include compaction, quartz overgrowths, authigenic clay minerals, carbonate cement and authigenic feldspar. Quartz overgrowths are present in the channel, floodplain, lagoonal and shallow marine facies but are more common in the channel facies. Also, the channell facies are characterised by pore-filling carbonate cement, grain-coating carbonate cement and carbonate replacement of grains. Carbonate cement, pore-filling clays and grain-coating clays occur in all the channel, floodplain, lagoonal and shallow marine facies. Fluvial channel and floodplain facies contain feldspar overgrowths which disappear in the lagoonal and shallow marine facies. Dissolution of unstable grains is observed in the channel, floodplain and lagoonal facies but is more abundant in the channel facies in the Narrabeen Group. Mechanical and chemical compaction is conspicuous in the channel facies but also occurs in the floodplain and lagoonal facies.

Reservoir quality and petrophysical properties of Cambrian sandstones and their changes during the experimental modelling of CO2storage in the Baltic Basin

The objectives of this study were (1) to review current recommendations on storage reservoirs and classify their quality using experimental data of sandstones of the Deimena Formation of Cambrian Series 3, (2) to determine how the possible CO2 geological storage (CGS) in the Deimena Formation sandstones affects their properties and reservoir quality and (3) to apply the proposed classification to the storage reservoirs and their changes during CGS in the Baltic Basin. The new classification of the reservoir quality of rocks for CGS in terms of gas permeability and porosity was proposed for the sandstones of the Deimena Formation covered by Lower Ordovician clayey and carbonate cap rocks in the Baltic sedimentary basin. Based on permeability the sandstones were divided into four groups showing their practical usability for CGS (‘very appropriate’, ‘appropriate’, ‘cautionary’ and ‘not appropriate’). According to porosity, eight reservoir quality classes were distinguished within these groups. The petrophysical, geochemical and mineralogical parameters of the sandstones from the onshore South Kandava and offshore E6 structures in Latvia and the E7 structure in Lithuania were studied before and after the CO2 injection-like alteration experiment. The greatest changes in the composition and properties were determined in the carbonate-cemented sandstones from the uppermost part of the South Kandava onshore structure. Partial dissolution of pore-filling carbonate cement (ankerite and calcite) and displacement of clay cement blocking pores caused significant increase in the effective porosity of the samples, drastic increase in their permeability and decrease in grain and bulk density, P- and S-wave velocity, and weight of the dry samples. As a result of these alterations, carbonate-cemented sandstones of initially ‘very low’ reservoir quality (class VIII), ‘not appropriate’ for CGS, acquired an ‘appropriate’ for CGS ‘moderate’ quality (class IV) or ‘very appropriate’ ‘high-2’ reservoir quality (class II). The permeability of the clay-cemented sandstones of ‘very low’ reservoir quality class VIII from the lower part of the E7 reservoir was not improved. Only minor changes during the alteration experiment in the offshore pure quartz sandstones from the E6 and E7 structures caused slight variations in their properties. The initial reservoir quality of these sandstones (‘high-1’ and ‘good’, classes I and III, respectively, in the E6 structure, and ‘cautionary-2’, class VI in the E7 structure) was mainly preserved. The reservoir sandstones of the Deimena Formation in the South Kandava structure had an average porosity of 21%, identical to the porosity of rocks in the E6 structure, but twice higher average permeability, 300 and 150 mD, respectively. The estimated good reservoir quality of these sandstones was assessed as ‘appropriate’ for CGS. The reservoir quality of the sandstones from the E7 offshore structure, estimated as ‘cautionary-2’ (average porosity 12% and permeability 40 mD), was lowest among the studied structures and was assessed as ‘cautionary’ for CGS. Petrophysical alteration of sandstones induced by laboratory-simulated CGS was studied for the first time in the Baltic Basin. The obtained results are important for understanding the physical processes that may occur during CO2 storage in the Baltic onshore and offshore structures.

Cathodoluminescence-revealed diagenesis of carbonates and feldspars in Cergowa sandstones (Oligocene), Outer Carpathians

Streszczenie Piaskowce cergowskie (dolny oligocen) charakteryzują się doskonałymi właściwościami fizyko- mechanicznymi, jednymi z najlepiej ocenianych wśród kruszyw budowlanych eksploatowanych w polskich Karpatach fliszowych. Obrazy katodoluminescencyjne piaskowców cergowskich przed- stawiają przede wszystkim węglany i skalenie, dlatego te dwie grupy minerałów zostały wybrane, aby zaprezentować i lepiej zrozumieć procesy odpowiedzialne za wyjątkowo silną cementację detrytycznych komponentów skały. Katodoluminescencja (CL) z zimną katodą umożliwia obserwację i interpretację cech diagenetycznych węglanów i skaleni, wizualizuje relacje przestrzenne między szkieletem ziarnowym i cementem, a tym samym pozwala na lepsze zrozumienie genezy wysokich wartości parametrów technicznych piaskowców cergowskich. W piaskowcach cergowskich roz- poznano za pomocą katodoluminescencji z zimną katodą trzy generacje cementu węglanowego (kalcytowego) wypełniającego przestrzenie międzyziamowe: kalcyt brązowy, kalcyt pomarańczowy i kalcyt żółty. Generacje te reprezentują spektrum od eo- przez mezo- po telodiagenezę. Skalenie zostały poddane procesom: albityzacji i zastępowania przez węglany. Dodatkowo, rozpuszczanie składników mineralnych wspomagane było ciśnieniem wywołanym kompakcją osadu. Proces roz- puszczania był na tyle słaby, że zniszczone zostały głównie brzeżne partie ziaren (korozja). Pomimo niedużej intensywności, proces ten był bardzo istotny w lityfikacji osadu. Przyczynił się on do zwiększenia powierzchni kontaktu korodowanych ziaren ze spoiwem. Kombinacja wspomnianych procesów spowodowała bardzo silną cementację składników piaskowców cergowskich, co przejawia się ich wysoką twardością i odpornością.

Factors controlling reservoir properties and hydrocarbon accumulation of lacustrine deep-water turbidites in the Huimin Depression, Bohai Bay Basin, East China

Lacustrine deep-water turbidite plays are a novel area for exploration in the Huimin Depression, Bohai Bay Basin. Turbidites in the Shang 847 block, a typical turbidite play in the Huimin Depression, provide an opportunity to study the factors controlling the reservoir properties and hydrocarbon accumulation in lacustrine turbidite sandstones. The reservoir quality of turbidite sandstones (very fine-grained, moderately to well sorted, mainly lithic arkose) in this study area are mainly controlled by the distribution patterns of carbonate cements and pseudomatrix. Significant inverse relationships exist between the volume of carbonate cement and both porosity and permeability of the turbidite sandstones. Carbonate cement is located preferentially near the margins of the sandstone bodies. Sandstones with distance from the sandstone–mudstone contact surface less than 0.7 m or with thickness less than 1.2 m are commonly tightly cemented (carbonate cement >15%) with low porosity and permeability (lower than 10% and 0.1 mD, respectively). The source of carbonate cement was most likely external, probably derived from the surrounding mudstone. Most pore-filling carbonate cements occurred during late diagenesis at burial depths greater than 2200 m. The petrophysical properties of turbidites have a positive relationship with the content of kaolinite and chlorite, but have a negative relationship with the content of illite. 2-D and 3-D reconstructions of non-oil bearing and oil-bearing layers indicate that dissolution of carbonate cement, feldspars and unstable rock fragments was more developed in oil-bearing layers than in non-oil bearing layers and hance oil-bearing layers have higher porosity and larger pore sizes. Petrophysical property appears to have a significant effect on the hydrocarbon accumulation in the turbidite sandstones. Sandstones with porosities lower than 9% and/or permeabilities lower than 0.78 mD are not prone to contain oil.

The influence of diagenetic alterations on porosity in the Triassic Narrabeen Group, Southern Sydney Basin, Australia

Petrography of the Triassic Narrabeen Group (Southern Sydney Basin of Australia) was described by thin section, scanning electron microscope and X-ray diffraction techniques. Sandstone composition includes mostly lithic grains with minor quartz, feldspar, mica and heavy minerals; litharenites and sublitharenites are common and quartzarenites are rare. Quartz includes monocrystalline and polycrystalline grains while the feldspar includes both K-feldspar and plagioclase. Volcanic, sedimentary and chert rock fragments are present. In thin section porosity is visible in all units, particularly the coarse-grained deposits. Secondary porosity is greater than the primary porosity. The diagenetic alterations include compaction, quartz overgrowths, authigenic clay minerals, carbonate cement and authigenic feldspar. Compaction occurred in all units during diagenesis. Both early and late diagenetic carbonate cement is important in the Narrabeen Group. Pore-filling carbonate cement reduced porosity whereas dissolution of carbonate resulted in secondary porosity. Authigenic clay minerals are widespread in all units filling pores and occurring as grain-coatings on detrital and authigenic grains. Where they coat quartz grains they preserve porosity by preventing growth of quartz overgrowths. Dissolution of unstable feldspar and lithic grains provided secondary porosity. In the Narrabeen Group, shale and siltstone are abundant in the Wombarra Claystone, Stanwell Park Claystone, Bald Hill Claystone and Newport Formation, forming lithological seals and confining layers. Minor sandstone beds occur in the Wombarra Claystone and contain more porosity. Medium- and coarse-grained sandstone is common in the Coalcliff Sandstone, Scarborough Sandstone and Bulgo Sandstone and shows low to moderate porosity. Thus, these sandstone units probably contain gas or water.

Composition, porosity, and reservoir potential of the Middle Jurassic Kashafrud Formation, northeast Iran

In the Kopet-Dagh Basin of Iran, deep-sea sandstones and shales of the Middle Jurassic Kashafrud Formation are disconformably overlain by hydrocarbon-bearing carbonates of Upper Jurassic and Cretaceous age. To explore the reservoir potential of the sandstones, we studied their burial history using more than 500 thin sections, supplemented by heavy mineral analysis, microprobe analysis, porosity and permeability determination, and vitrinite reflectance. The sandstones are arkosic and lithic arenites, rich in sedimentary and volcanic rock fragments. Quartz overgrowths and pore-filling carbonate cements (calcite, dolomite, siderite and ankerite) occluded most of the porosity during early to deep burial, assisted by early compaction that improved packing and fractured quartz grains. Iron oxides are prominent as alteration products of framework grains, probably reflecting source-area weathering prior to deposition, and locally as pore fills. Minor cements include pore-filling clays, pyrite, authigenic albite and K-feldspar, and barite. Existing porosity is secondary, resulting largely from dissolution of feldspars, micas, and rock fragments, with some fracture porosity. Porosity and permeability of six samples averages 3.2% and 0.0023 mD, respectively, and 150 thin-section point counts averaged 2.7% porosity. Reflectance of vitrinite in eight sandstone samples yielded values of 0.64–0.83%, in the early mature to mature stage of hydrocarbon generation, within the oil window. Kashafrud Formation petrographic trends were compared with trends from first-cycle basins elsewhere in the world. Inferred burial conditions accord with the maturation data, suggesting only a moderate thermal regime during burial. Some fractures, iron oxide cements, and dissolution may reflect Cenozoic tectonism and uplift that created the Kopet-Dagh Mountains. The low porosity and permeability levels of Kashafrud Formation sandstones suggest only a modest reservoir potential. For such tight sandstones, fractures may enhance the reservoir potential.

Depositional and Diagenetic Controls on Production in Morrow Valley Fills, Central State Line Area, Colorado/Kansas: ABSTRACT

Production from wells in Pennsylvanian Morrow valley-fill deposits along the central state-line trend of Colorado and Kansas is controlled by sedimentary facies and diagenesis. The reservoir sandstone units fill paleovalleys incised up to 25 m (80 ft) into the underlying marine shale and limestone. In the state line area, multiple valley incisions occurred, superimposing valley fills with distinct reservoir characteristics. Most of the production comes from a coarse-grained permeable fluvial deposit that filled a paleovalley incised into an underlying fine-grained and low-permeability estuarine valley fill. The depositional and diagenetic distinctions between these two valley fills explain most of the complexity observed along this 2-km (1-mi) wide, 32-km (20-mi) long trend. The coarse-grained younger deposit has large, open pore throats generated by the dissolution of pore-filling carbonate cement. Migration of fine kaolinite booklets through the coarse deposit occurs without blocking pore throats because the pore-throat radii are larger than the clay particles. Although capable of good initial production, the fine-grained, older, more estuarine valley fill is subject to blockage of pore throats by particulate kaolinite, which causes precipitous declines in production. The fine-grained valley fill can be differentiated from the coarse (highly productive) valley fill by integrating core and log data.more » Once identified, the two deposits can be mapped and used predictively as the field continues to develop to the north and south.« less

Patterns of diagenesis in the Sherwood Sandstone Group (Triassic), United Kingdom

AbstractThe Triassic Sherwood Sandstone Group comprises a complex of continental red beds deposited by a major fluvial system flowing dominantly down a northerly inclined palaeoslope. Sedimentation took place in several distinct, tectonically active basins with varying maximum burial depths, ranging from shallow (<1 km) to deep (>3 km). Despite proximal to distal variations in stream type, a distinct suite of early diagenetic events can be recognized throughout all the depositional basins, which is related to the depositional environment. These events are best preserved in those basins with shallow burial histories, and show many similarities to the processes recorded from modern red beds of the Sonoran Desert, Baja California, although a more advanced grade of diagenesis has been reached in the Sherwood Sandstone. On the margins of the Irish Sea Basin in Cumbria, where burial was shallow, these early diagenetic textures are well preserved. The detrital grains underwent changes aimed at reaching equilibrium with the near-surface chemical environment. Unstable silicates were dissolved or replaced and the released ions were capable of precipitating authigenic phases, typically mixed-layer illite-smectite, K-feldspar, non-ferroan carbonates and hematite. Lateral variations in the early diagnetic assemblages reflect chemical and spatiotemporal changes ofinterbasin depositional and diagenetic environments. Deeply buried equivalents in the Irish Sea reached a maximum burial depth in excess of 3 km towards the end of the Mesozoic. Superimposed on the early diagenetic fabric are a series of depth-related changes. In the absence of early cements, compaction reduced porosity to low levels. Mixed-layer illite-smectites were converted to highly crystalline illites with low Fe and Mg contents. Early framework-preserving non-ferroan carbonates were extensively dissolved, generating widespread secondary porosity. Late pore-filling carbonate cements comprise ferroan dolomites and ankerites with compositions up to Ca(Ca0·02Mg0·43Fe0·53Mn0·02). Following late Mesozoic burial, inversion of the Triassic basins resulted in the re-exposure of basin margin sequences and shallow burial basins. Modern groundwaters are typically very dilute, with essentially neutral pH, and are capable of dissolving carbonate, sulphate and halite cements. In the Wessex Basin, however, the Triassic sandstones of the South Devon aquifer have been extensively modified by the percolation of post-inversion acidic groundwaters. These low-pH waters have resulted in thein situbreakdown of feldspar to produce abundant authigenic kandite, widespread dissolution of carbonate cements and removal of early-formed iron oxide cements. In contrast, the concealed, deeply buried eastern margin of the Wessex Basin exhibits a diagenetic evolution resultant of deep burial and has no development of authigenic kandite.

Geochemical hydrocarbon exploration — a new/old exploration tool

The search for petroleum has evolved into a highly sophisticated technology where today almost every scientific discipline known is being brought to bear upon the endeavour. Yet, the use of geochemical hydrocarbon exploration remains a peripheral exploration tool. The trend toward scientific integration has led the petroleum explorationist to the point of being a specialist. It would seem that our petroleum scientists have focussed their interests mainly on the investigation of principles and less on their ultimate purpose of discovering new and larger oil and gas reserves. So, it is not by chance, that leading geochemists have been speaking more and more freely of the necessity to integrate our tools of exploration and thereby do a better job. The theoretical basis for hydrocarbon geochemistry is complex, and, as with all exploration tools, the problems and difficulties of interpreting the data will never be completely eliminated. This article considers the importance of using the ΔC method in geochemical hydrocarbon exploration which has been employed successfully for over 40 years. The addition of carbon-isotope ratios and trace-element analysis to this method has added a new dimension to geochemical hydrocarbon exploration. The theoretical basis of the ΔC method has been presented earlier by the author and will only be touched upon briefly here. Very simply, the basis of all geochemical hydrocarbon exploration is based on the much debated premise that the lighter hydrocarbon gases and their components migrate vertically from a trap through the overlying sedimentary pile to the surface. Upon reaching the surface, through oxidation, they leave their signatures in one form or another that can be detected by physicochemical methods. These physicochemical signatures are discernable as “geochemical haloes”. From soil samples, collected from 2–3 m deep, what is measured is the result of absorption and adsorption by soil particles that are altered to CO 2 by oxidation and form a unique, stable, carbonate system with the surface and near-surface material. This is unlike other carbonate systems and when subjected to a differential thermal technique, dissociates into CO 2 surface material is cumulative and indicates where maximum hydrocarbon leakage has taken place over the life span of the material sampled. It is durable and unaffected by pressure and temperature variation or recent hydrocarbon contamination. Values are expressed in terms of millivolts which are proportional to the CO 2 given off by the dissociation of the carbonate system under standard conditions. Frequency curves are constructed for all values for the determination of significant contour levels above the normal geochemical background for mapping. After significant ΔC anomalies are located, they can be further verified by use of carbon-isotope ratios. As methane migrates to the surface from underlying hydrocarbon accumulations, there is a progressive selection or fractionation that causes enrichment of the carbon-13 isotope. The methane, thus reaching the near-surface, is isotopically lighter. When oxidized in accordance with the equation CH 4 + 2O 2 → 2H 2O + CO 2, the carbon having been converted to carbon dioxide, is taken up in the pore-filling carbonate cements that are found in the near-surface soils and sediments. High carbon dioxide values (ΔC) in the geochemical halo are related the δ 13C carbon-isotope ratios from underlying hydrocarbon accumulations. This is observed over fields containing hydrocarbon accumulations where δ 13C values in the pore-filling carbonate cements become increasingly negative (lighter) toward the crests of traps (i.e. exhibiting lower ΔC values). This indicates enrichment of 12C relative to the PDB standard. Whereas, positive values of δ 13C indicate depletion in 12C or enrichment in 13C (i.e. exhibiting higher ΔC values away from the crests of the traps). The observed ΔC anomalies and δ 13C anomalies leave an indelible pattern in the near-surface sediments and soils which are herein referred to as geochemical hydrocarbon haloes. Trace-element associations, that form organometallic compounds, are found “haloed” or concentrated over or around underlying hydrocarbon reservoirs. These associations seem to have occurred from vertically migrating methane that has acted as a “carrier” sweeping up the trace elements on the pathways to the surface. Vanadium, nickel, chromium, iron, cobalt, copper, manganese, strontium, barium are various trace element ratios seen to also halo and indicate subsurface hydrocarbon accumulations. An example presented from the Ocho-Juan Field, a producing reef field, located in Scurry and Fisher Counties, Texas shows that the combination of ΔC, δ 13C and trace-element analysis from near-surface soil sampling is a significant step forward in improving geochemical hydrocarbon exploration methods.