Rock Core Samples Research Articles

Petroleum systems in the offshore Xihu Basin on the continental shelf of the East China Sea

Xihu Basin is one of the Cenozoic sedimentary basins within the continental shelf of the East China Sea, within which eight oil and gas fields and four hydrocarbon-bearing structures have been found. Our systematic analysis of potential petroleum systems in the basin has identified the Eocene Pinghu Formation as the most important source rock in the basin. The Eocene Pinghu Formation consists of mudstone and coal deposited in an embayment and tidal-flat environment and is characterized as containing type III kerogen. The Pinghu Formation is mature and, at the present time, is within the oil and wet-gas windows with determined vitrinite reflectance values in the range of 0.55–2.2% measured vitrinite reflectance (Rom). Modeling results suggest that the main stage of hydrocarbon expulsion occurred during the Miocene. The main reservoir consists of fine-grained sandstones of the Huagang and Pinghu formations deposited within shallow lacustrine and estuarine environments. The measured porosity from core samples of reservoir rock ranges from 10 to 35%, whereas permeability values range from 1 to 4000 md; both porosity and permeability decrease with depth. The mudstones of the upper Huagang and Longjing formations also occur as regional seals, which developed overpressure zones as determined by two-way sonic transit times. The overburden includes all the Oligocene, Miocene, Pliocene, and Quaternary strata. The hydrocarbon traps in the basin are mainly structural traps, including faulted blocks, faulted structural noses, and anticlines. Most traps were developed at the ends of the Eocene or Oligocene. Hydrocarbons produced from the Xihu Basin are predominantly natural gases with minor amounts of crude oil. The crude oil in the basin is characterized by a low density, low sulfur, low wax, low vanadium/nickel ratio, a low resin and asphaltene content, and a high proportion of saturated hydrocarbons. The natural gas in the basin is composed predominantly of methane, with an average C1/C1–5 ratio of 0.89 and a C1/C2–5 ratio of 8.6. Two petroleum systems have been identified in the basin: the known Pinghu-Huagang and the hypothetical Huagang-Huagang systems. The Pinghu-Huagang is the major petroleum system and most important for oil and natural gas exploration in the basin.

Metal sources of the Navan carbonate-hosted base metal deposit, Ireland: Nd and Sr isotope evidence for deep hydrothermal convection

Nd and Sr isotope analyses are presented for gangue mineral samples from the giant carbonate-hosted Navan Zn–Pb deposit, Ireland, and for rocks from which Navan metals may have been derived. Analysis of gangue minerals spanning the Navan paragenetic sequence reveals systematic evolution in the composition of the mineralising fluid. Early fluid represented by replacive dolomite exhibits the lowest initial 87Sr/86Sr ratio (0.7083–0.7086), closest to that of the host limestone and to Lower Carboniferous seawater, and the highest 143Nd/144Nd ratio (0.51161–0.51176). Later generations of dolomite, barite and calcite, which encompass sulphide precipitation, have higher initial 87Sr/86Sr ratios (maximum 0.7105) and lower initial 143Nd/144Nd ratios (minimum 0.51157). All samples have initial Nd isotope ratios that are too low to have been acquired only from the host limestone. Drill core samples of presumed Ordovician volcanic and sedimentary rocks from beneath the Navan orebody have 143Nd/144Nd and 87Sr/86Sr ratios at the time of mineralisation of 0.51184–0.51217 and 0.7086–0.7138, respectively. The data are interpreted to indicate mixing of sulphide-rich, limestone-buffered brine, with a metal-bearing hydrothermal fluid, which had passed through sub-Carboniferous rocks, consistent with published fluid inclusion and S isotope data. The 143Nd/144Nd ratio of this basement-derived fluid is too low to have been imparted by flow through the Devonian Old Red Sandstone, as required in models of regional fluid flow in response to Hercynian uplift. Irrespective of whether such regional fluid flow occurred, the hydrothermal Nd must have been derived from sub-Devonian rocks. These conclusions broadly support the hydrothermal convection cell model in which brines, ultimately of surface origin, penetrated to a depth of several kilometres, leaching metals from the rocks through which they passed. The data also support increasing depth of penetration of convection cells with time. Metals were subsequently precipitated in carbonate rocks at sites of mixing with cooler, sulphide-rich fluids. However, comparison of the Navan hydrothermal gangue Nd–Sr isotope data with data from Lower Palaeozoic rocks strongly suggests that the latter cannot alone account for the “basement” signature. As the Navan deposit lies immediately north of the Iapetus Suture, this suggests that the Laurentian margin includes Precambrian basement.

Stress-Dependent Fluid Flow and Permeability in Fractured Media: from Lab Experiments to Engineering Applications

Permeability is a physical property in rocks of extreme importance in energy engineering, civil and environmental engineering, and various areas of geology. Early on, fractures in fluid flow models were assumed to be rigid. However, experimental research and field data confirmed that stress-deformation behavior in fractures is a key factor governing their permeability tensor. Although extensive research was conducted in the past, the three-dimensional stress-permeability relationships, particularly in the inelastic deformation stage, still remain unclear. In this paper, laboratory experiments conducted on large concrete blocks with randomly distributed fractures and rock core samples are reported to investigate fluid flow and permeability variations under uniaxial, biaxial and triaxial complete stress-strain process. Experimental relationships among flowrate, permeability and fracture aperture in the fractured media are investigated. Results show that the flowrate and stress/aperture exhibit “cubic law” relationship for the randomly distributed fractures. A permeability-aperture relationship is proposed according to the experimental results. Based on this relationship, stress-dependent permeability in a set of fractures is derived in a three-dimensional domain by using a coupled stress and matrix-fracture interactive model. A double porosity finite element model is extended by incorporating such stress-dependent permeability effects. The proposed model is applied to examine permeability variations induced by stress redistributions for an inclined borehole excavated in a naturally fractured formation. The results indicate that permeability around underground openings depends strongly on stress changes and orientations of the natural fractures.

Petrophysical inversion of borehole array-induction logs: Part II — Field data examples

We describe the application of Alpak et al.’s (2006) petrophysical inversion algorithm to the interpretation of borehole array induction logs acquired in an active North American gas field. Layer-by-layer values of porosity and permeability were estimated in two closely spaced vertical wells that penetrated the same horizontal rock formation. The wells were drilled with different muds and overbalance pressures, and the corresponding electromagnetic induction logs were acquired with different tools. Rock-core laboratory measurements available in one of the two wells were used to constrain the efficiency of gas displacement by water-based mud during the process of invasion. Estimated values of porosity and permeability agree well with measurements performed on rock-core samples. In addition to estimating porosity and permeability, the petrophysical inversion algorithm provided accurate spatial distributions of gas saturation in the invaded rock formations that were not possible to obtain with conventional procedures based solely on the use of density and resistivity logs.

Spatial correlation structures of fracture systems for deriving a scaling law and modeling fracture distributions

An understanding of fracture systems in geological media requires two fracture properties clarification: fracture distributions and the scaling law of fracture systems over different scales. To investigate fracture systems in different scales, we used remotely sensed images from LANDSAT and SPOT satellites, borehole-fracture data in two drilling directions, and a thin-section of rock core sample. The eastern part of the Tohoku district, northeastern Japan, overlain by Cretaceous granite and a Jurassic accretionary complex, was chosen as the study area. Assuming that part of the line features detected in satellite and thin-section images correspond with real fractures, the spatial correlation structures of fractures were clarified by semivariograms to identify an alternative scaling law. For this we focused on joint's line density along boreholes, area density of linear features, and directional relations of strikes between a fracture pair to produce semivariograms of densities and cross-semivariograms of the strikes transformed into binary data sets. The same model independent of the scales could approximate the semivariograms of each parameter. Spatial correlation distances were obtained from the range of the semivariogram model. An important characteristic was that linear trends between representative and correlation distances of fractures were found in log–log plots. Scaling law can contribute to estimating correlation distances at arbitrary fracture scales. A conditional distribution modeling at the borehole-fracture scale was presented as an application of the semivariograms obtained by the three steps: generation of fracture-density map by a sequential gaussian simulation, assignment of strikes to each simulated fracture, and connection of fractures considering distances and differences in strikes between a closely located fracture pair. Additionally, nonconditional simulation at arbitrary fracture scales and changes of permeabilities with scales using a permeability tensor analysis are discussed.

A guarded cylindrical capacitor for the non-destructive measurement of hard rock core samples

The novel design of a flexible guarded capacitor which conforms to a core sample's cylindrical surface is presented. The proposed device allows non-destructive measurements of hard rock samples produced by diamond drilling, and has numerous advantages over previous methods. No material is lost to sample preparation and a detailed characterization of the entire core, including inclusions and transitions between rock types, may be performed. A detailed methodology for the rapid and cost-effective construction of a robust capacitor is given. Guidelines for its operation to achieve repeatable and accurate measurements of the complex dielectric constant of samples of varying homogeneity in the 1–25 MHz frequency range are presented.

Metamorphic petrology and geochemistry of the Sarkadkeresztúr Basement-High with special regard to orthogneiss (Tisza Mega-unit, SE Hungary)

Core samples of metamorphic rocks from the Sarkadkeresztur Basement High (Bekes-Codru Unit, Tisza Mega-unit) were examined, consisting mainly of two-feldspar augengneiss, garnet-andalusite-staurolite-bearing micaschist and paragneiss. The protolith of the plagioclase-microcline-augengneiss was found to be S-type granite (syenogranite) of unknown age. According to the major element analyses the protoliths of the studied metagranitoids of this basement can be classified as a syenogranite of metaluminous and peraluminous character displaying subalkaline affinity and a calcic granitic composition. The REE patterns are characterized by a slight enrichment of the light REEs and negative Eu and Yb anomalies. On the basis of discrimination diagrams for major and trace elements, the studied rocks were formed in a syn-collisional (continent-continent collision zone) tectonic environment. The low-pressure amphibolite facies metamorphism, which affected this basement, can be assigned to the Variscan.

Geophysical modeling of the northern Appalachian Brompton-Cameron, Central Maine, and Avalon terranes under the New Jersey Coastal Plain

A regional terrane map of the New Jersey Coastal Plain basement was constructed using seismic, drilling, gravity and magnetic data. The Brompton-Cameron and Central Maine terranes were coalesced as one volcanic island arc terrane before obducting onto Laurentian, Grenville age, continental crust in the Taconian orogeny [Rankin, D.W., 1994. Continental margin of the eastern United States: past and present. In: Speed, R.C., (Ed.), Phanerozoic Evolution of North American Continent-Ocean Transitions. DNAG Continent-Ocean Transect Volume. Geological Society of America, Boulder, Colorado, pp. 129–218]. Volcanic island-arc rocks of the Avalon terrane are in contact with Central Maine terrane rocks in southern Connecticut where the latter are overthrust onto the Brompton-Cameron terrane, which is thrust over Laurentian basement. Similarities of these allochthonous island arc terranes (Brompton-Cameron, Central Maine, Avalon) in lithology, fauna and age suggest that they are faulted segments of the margin of one major late Precambrian to early Paleozoic, high latitude peri-Gondwana island arc designated as “Avalonia”, which collided with Laurentia in the early to middle Paleozoic. The Brompton Cameron, Central Maine, and Avalon terranes are projected as the basement under the eastern New Jersey Coastal Plain based on drill core samples of metamorphic rocks of active margin/magmatic arc origin. A seismic reflection profile across the New York Bight traces the gentle dipping (approximately 20 degrees) Cameron's Line Taconian suture southeast beneath allochthonous Avalon and other terranes to a 4 sec TWTT depth (approximately 9 km) where the Avalonian rocks are over Laurentian crust. Gentle up-plunge (approximately 5 degrees) projections to the southwest bring the Laurentian Grenville age basement and the drift-stage early Paleozoic cover rocks to windows in Burlington Co. at approximately 1 km depth and Cape May Co. at approximately 2 km depths. The antiformal Shellburne Falls and Chester domes and Chain Lakes-Pelham dome-Bronson Hill structural trends, and the synformal Connecticut Valley-Gaspe structural trend can be traced southwest into the New Jersey Coastal Plain basement. A Mesozoic rift basin, the “Sandy Hook basin”, and associated eastern boundary fault is identified, based upon gravity modeling, in the vicinity of Sandy Hook, New Jersey. The thickness of the rift-basin sedimentary rocks contained within the “Sandy Hook basin” is approximately 4.7 km, with the basin extending offshore to the east of the New Jersey coast. Gravity modeling indicates a deep rift basin and the magnetic data indicates a shallow magnetic basement caused by magnetic diabase sills and/or basalt flows contained within the rift-basin sedimentary rocks. The igneous sills and/or flows may be the eastward continuation of the Watchung and Palisades bodies.

Permeability imaging in granitic rocks based on surface resistivity profiling

In order to image the distribution of permeability in granitic rocks, we carried out two-dimensional (2D) resistivity profiling, together with in-situ permeability tests, electrical logging of boreholes, and resistivity measurements of rock core samples in a laboratory. Based on the electrical logging and in-situ permeability data from boreholes, we obtained empirical equations which relate resistivity and permeability of the granitic rocks in the area studied. We then applied the empirical equation to a 2D resistivity section, to produce a 2D permeability section of the granitic rocks. In this paper, we present details of the field data and of the procedure for conversion from the resistivity section to a permeability section. The observed relationship between resistivity and permeability of the rocks is also discussed.

Fracture toughness measurements on igneous rocks using a high-pressure, high-temperature rock fracture mechanics cell

A sound knowledge of mechanical properties of rocks at high temperatures and pressures is essential for modelling volcanological problems such as fracture of lava flows and dike emplacement. In particular, fracture toughness is a scale-invariant material property of a rock that describes its resistance to tensile failure. A new fracture mechanics apparatus has been constructed enabling fracture toughness measurements on large (60 mm diameter) rock core samples at temperatures up to 750°C and pressures up to 50 MPa. We present a full description of this apparatus and, by plotting fracture resistance as a function of crack length, show that the size of the samples is sufficient for reliable fracture toughness measurements. A series of tests on Icelandic, Vesuvian and Etnean basalts at temperatures from 30 to 600°C and confining pressures up to 30 MPa gave fracture toughness values between 1.4 and 3.8 MPa m 1/2. The Icelandic basalt is the strongest material and the Etnean material sampled from the surface crust of a lava flow the weakest. Increasing temperature does not greatly affect the fracture toughness of the Etnean or Vesuvian material but the Icelandic samples showed a marked increase in toughness at around 150°C, followed by a return to ambient toughness levels. This material also became tougher under moderate confining pressure but the other two materials showed little change in toughness. We describe in terms of fracture mechanics probable causes for the changes in fracture toughness and compare our experimental results with values obtained from dike propagation modelling found in the literature.

Interrelations Between Thermal Conductivity and Other Physical Properties of Rocks: Experimental Data

— A new non-contact and non-destructive optical scanning instrument provided a large number of high-precision measurements of thermal conductivity tensor components in samples of sedimentary and impact rocks, as well as new insights into interrelations between thermal conductivity and other physical properties. More than 800 core samples (dry and fluid-saturated) of sedimentary rocks from different Russian oil-gas deposits and impact rocks from the well “Nordlingen 1973” drilled in the Ries impact structure (Germany) were studied using optical scanning technology. It was established that the thermal conductivity parallel to the stratification is more informative for petrophysical investigations than the thermal conductivity perpendicular to the layering. Different approaches were developed to estimate porosity, permeability, pore space geometry, and matrix thermal conductivity with a combination of thermal conductivity measurements in dry and fluid-saturated samples and mathematical modelling. These approaches allow prediction of the rock porosity and permeability and their spatial distribution along a well using thermal conductivity measurements performed with the optical scanning instrument directly applied to cores. Conditions and constraints for using Lichtenecker-Asaad's theoretical model for the estimation of porosity and thermal conductivity of sedimentary rocks were determined. A correlation between thermal conductivity and acoustic velocity, porosity, density, and electric resistivity of impact rocks was found for different rock types. New relationships between permeability, electrical and thermal conductivity found for sedimentary rocks are described.

In-situ Measurements of Cutter Forces on Boring Machine at Äspö Hard Rock Laboratory Part II. Characteristics of Cutter Forces and Examination of Cracks Generated

By means of spectral analysis, the measured normal forces, tangential forces, and side forces acting on two button cutters on the boring machine in Aspo Hard Rock Laboratory were analysed and the basic characteristics of the cutter forces were determined. After the measurements of the cutter forces, some rock core samples were taken from the bottom and the wall of the test borehole. These samples were cut, polished, and examined by means of a Scanning Electron Microscope (SEM). The lengths of the major cracks in the rock samples were measured, and a relation between the length of the median cracks and the relevant cutter forces was investigated.

４．非線形力学 非均質性が岩盤構造物の挙動に及ぼす影響と設計に関する研究

It is necessary to construct the analytical method that can reproduce real phenomena accurately so as to discuss the safety of rock structures withheterogeneity, and to design them economically andrationally. Thus, in this study, the design flow of rock structures with heterogeneity is proposed and the effects of heterogeneity on the behaviors of rock structures are di scussed. In the design flow, at frrst, the constitutive equation of the rock is determined by the comparison between a analysis and a laboratory experiment with rock core sample. Next, the numerical examination with the determined constitutive equation is carried to investigate the safety and durabihty of rock structures. Trough the comparison to experiments, the small region of heterogeneity has little influence on the macroscopic behaviors of rock structure

Distribution of microorganisms in the subsurface of the manus basin hydrothermal vent field in Papua New Guinea.

The distribution of microorganisms in the subsurfaces of hydrothermal vents was investigated by using subvent rock core samples. Microbial cells and ATP were detected from cores taken at depths of less than 99.4 and 44.8 m below the seafloor (mbsf), respectively. Cores from various depths were incubated anaerobically with a heterotrophic medium. Growth at 60 and 90 degrees C was ascribed to a Geobacillus sp. in the 448.6- to 99.4-mbsf cores and a Deinococcus sp. in the 64.8- to 128.9-mbsf cores, respectively, based on the 16S ribosomal DNA analysis.

Unusual N-alkane distributions in extracts from marine carbonate rocks at high levels of maturity and overmaturity

N-alkanes in extracts from possible carbonate source rocks of the Lower Ordovician Majiagou Formation in the central gas field of the Shanganning Basin and the Upper Sinian Dengying Formation in the Weiyuan gas field of the Sichuan Basin, are characterized by bimodal distributions with the dominant carbon numbers in the range C17-C21 and C25-C29. In most samples, the lower carbon number components are present in greater abundance than the higher carbon number ones. Most samples contain significant concentrations of waxy hydrocarbons (C22+) with C21-/C22+ ratios between 0.50 to 3. 16, and an average value of 1. 34. The n-alkanes in extracts of outcrops and shallow core samples of Upper Proterozoic and Lower Palaeozoic carbonate rocks in the western and southern parts of the North China Basin are of unimodal distributions dominated by n-alkanes maximizing in the C25-C29 range. These extracts have very high concentrations of waxy hydrocarbons with C21-/C22+ ratios all < 1. 0, ranging from 0. 14 to 0. 90 and averaging 0. 36. All of the extracts have a marked odd/even predominance in the high molecular weight n-alkane range regardless of whether they are from shallow or deep cores or out-crop samples. Simulation experiments were performed using typical sapropelic-type kerogens from the immature Sinian Lower Xiamalin Formation carbonate-rich shales collected at Jixian, Hebei Province, North China Basin, and the contemporary microplanktonic blue-green algaeSpirulina subsala. Results indicate that the unusual distribution of n-alkanes in the extracts of Upper Proterozoic and Lower Palaeozoic carbonate rocks possibly originated from algae in the source rocks at high levels of maturity and overmaturity.

Recovery of Original Organic Parameters of the the Outcropping Source Rocks from South China

Two samples from outcrops and six core samples of the clay source rocks from the Shiwanshan and Baise Basins were analysed in order to study the variation of selected organic parameters. A comparison of the samples taken from the outcrops with core samples was then performed. The results indicate a significant alteration of the organic matter at and near the surface. A depletion of about 70% TOC and extract yields was seen. The depth influenced by oxidation was found to be about 50 m. The most significant depletion of the organic matter occurred within 16 m.

Estimation of Cavability by Using a Block Model and Fuzzy Sets.

This paper presents a classification method for estimating the cavability class of an ore body, involving four evaluation parameters: rock quality designation (RQD), modified joint spacing, Js, modified joint set number, Jf, and shear property of joints, Jφ. Geostatistical tools and a block modeling techniques are used to estimate the value of each parameter. Fuzzy sets are applied to estimate the cavability class of the rock mass block. The goal of this research is to develop a three dimensional (3D) block cavability model by incorporating these rock mass rating parameters. In this research work, the rock mass characterization data were derived from the rock core samples and the drift logging of a copper mine in north China.

Laboratory Study and Field Matching of Matrix Acidizing of Petroleum Reservoir Rocks

In this paper, a laboratory method was developed to contaminate rock core samples with drilling fluids and test formation damage caused for these cores. The surface of the contaminated core samples was treated with different acid solutions to simulate matrix acidizing, remove damage and restore the original permeability. The acid volume and concentration, reaction speed and reaction products of different acid types and acid mixtures used in acidizing are presented. The amount of acid injection volume required to remove the formation damage near the wellbore region was calculated for different acid concentrations and acid solutions.Field data from some fields in one of the Arabian Gulf Countries were collected and analyzed and matched with the laboratory results obtained. Moreover other processes to remove or to minimize formation damage are mentioned. It was found that 0.8 Pore Volume (PV) of 15% HC1 is adequate to remove formation damage and to restore the permeability or 1.3 PV of 12% HC1; 0.93 PV of a mixture of 15% HC1 and 15% formic acid; 1.1 PV of a mixture of 15% HC1 and 15% acetic acid; 2.3 PV of 10% formic acid or 3.5 PV of 10% acetic acid. Matching the results obtained with the collected field data shows that the acid injection volume is overestimated which increases the operation cost and affects the economical factors of the operation.

Soil and rock strength parameters of the Shiraidake Landslide, Japan

The Shiraidake area of northwest Kyushu has an extensive distribution of landslides. A series of undrained triaxial tests and unconfined compression tests were carried out to investigate the variation of strength properties in a selected borehole from the Shiraidake Landslide. It is a translational landslide (called the Hokusho-type in Japan) and is composed of the Early Tertiary and Quaternary sedimentary rocks that are prone to rapid weathering. Core rock samples and recompacted soil samples were used for the study. The results provided the fundamental characteristics of soil and rock under the triaxial and uniaxial tests, and indicated that the undrained behaviour of tested soils generally depends on the pre-shear consolidation pressure and dry density. In addition, the type of material used and the tests performed are important factors that influence the soil and rock strength.

MRI of pore size distributions in fault sealing oil reservoir rock core samples by cryoporometric filtering techniques

MRI of pore size distributions in fault sealing oil reservoir rock core samples by cryoporometric filtering techniques