Scientific Borehole Research Articles

Structure of a silica diagenetic transformation zone: the Gjallar Ridge, offshore Norway

AbstractThree‐dimensional seismic data from the Gjallar Ridge were used together with X‐ray diffraction data, scientific boreholes and wireline logs to analyse the seismic structure of a silica diagenetic transformation zone. The following features were identified: (i) an interval some 150 to 300 m thick that contains anomalously high‐amplitude reflections; (ii) a strong reflection event at the top of this interval which cross‐cuts stratigraphy, interpreted as the transformation boundary between opal‐A‐rich and opal‐CT‐rich sediment; (iii) amplitude variations on stratigraphic reflections within the interval, attributed to variations in the proportions of opal‐A and opal‐CT; and (iv) a second, deeper, cross‐cutting reflection event within the interval, interpreted as the transformation boundary between opal‐CT‐rich and quartz‐rich sediment. The base of the interval containing the anomalously high‐amplitude reflections is interpreted as a stratigraphic reflection demarcating the base of the silica‐rich strata. On a stratigraphic reflection within the interval of high amplitudes, roughly circular regions of anomalously high amplitude with diameters of 0·8 to 2·5 km are separated by lower amplitude regions. This pattern is similar to the cells previously identified at the opal‐A to opal‐CT transformation boundary and probably results from more complete transformation of opal‐A to opal‐CT. All of these observations provide the first recognition from seismic data that silica diagenetic transformations are not always narrow boundaries represented by single cross‐cutting seismic reflection events, as implied previously, but can be heterogeneous and hundreds of metres in thickness, as observed at outcrop.

Consistent trends in the true triaxial strength and deformability of cores extracted from ICDP deep scientific holes on three continents

An extensive true triaxial testing program was carried out on core samples from three ICDP-sponsored deep scientific boreholes, KTB (Germany), SAFOD (United States), and TCDP (Taiwan). The three rocks differ in the processes that formed them and in many of their mechanical properties. However, all three rocks exhibited similar failure mechanism, in which induced or reopened microcracks are primarily aligned with the σ 1-σ 2 plane, and the developed fault is steeply inclined in the σ 3 direction. Rock strength in all tested rocks increases with σ 2 when σ 3 is kept constant. Thus, the common Mohr-type criteria, which ignore the effect of σ 2, typically underestimate rock strength. Rather a 3D criterion that involves all three principal stresses represents well experimental results. Fracture plane slope for the same σ 3 steepens as σ 2 rises, contrary to Mohr-type criteria. With respect to deformation, the onset of dilatancy increases with σ 2. In conclusion, true triaxial tests conducted on cores from three scientific boreholes, revealed important details of mechanical behavior not otherwise observed in conventional triaxial tests. In addition, they show mechanical behavior similarities as related to σ 2 effect regardless of rock type.

Physical properties of tuffs from a scientific borehole at Alban hills volcanic district (central Italy)

Recent seismic swarms and hydrothermal activity suggest that the Quaternary volcanic complex of the Alban Hills may pose a threat to the city of Rome. A 350 m scientific borehole was therefore drilled into this volcanic area to elucidate its inner structure for the first time. Wire-line logs were run in the borehole in order to characterize the physical properties of the rocks and their variations with depth. In particular, a detailed sonic log was run to measure the P-wave velocity from the well-head down to 110 m. To further investigate velocity changes, we carried out laboratory measurements of P and S elastic wave velocities and fluid permeability at effective pressures up to 70 MPa during both increasing and decreasing pressure cycles on selected core samples representative of the main volcanic units. Specifically, we studied samples from two pyroclastic units representative of two classes of volcanic deposits that are representative of the whole succession: (i) a coarse-grained, well-lithified facies (Pozzolane Rosse unit), containing abundant mm-to-cm lava clasts and crystals; and (ii) a fine-grained, matrix-supported pyroclastic deposit (Tufo Pisolitico di Trigoria unit), with rare lithic lava clasts and sparse pumice. Elastic wave velocities reveal significant differences between units and indicate how, within the same lithology, the different degree of lithification and presence of clasts can affect significantly physical property values. The mean laboratory value of the P-wave velocity for Pozzolane Rosse and Tufo Pisolitico di Trigoria units is respectively of 3.75 and 3.2 km/s at an effective pressure equivalent to that at the depth at which the sonic velocity was measured. Under increasing effective pressure a profound influence on the transport properties is observed. Permeability ranges from the order of 10 − 18 m 2 for the Pozzolane Rosse unit to the order of 10 − 15 m 2 for the Tufo Pisolitico di Trigoria unit, in good agreement with the shallow aquifer circulating in the shallower units.

Cyclostratigraphic dating in the Lower Badenian (Middle Miocene) of the Vienna Basin (Austria): the Baden-Sooss core

The scientific borehole Baden-Sooss penetrates a succession of Badenian (Langhian, Middle Miocene) sediments at the type locality of the Badenian, the old brickyard Baden-Sooss in the Vienna Basin. The sedimentary succession of the 102-m-cored interval consists of more than 95% bioturbated, medium-to-dark gray marly shales with carbonate contents between 11 and 25% and organic carbon contents between 0.35 and 0.65%. Biostratigraphic investigations on foraminifera (mainly lower part of Upper Lagenid Zone) and calcareous nannoplankton (standard zone NN5) indicate an early Badenian (Langhian) age. Cycles in carbonate content, organic carbon content, and magnetic susceptibility have been identified by power spectra analysis. Correlations between the three variables are extremely significant. Using cross-correlation, periods around 40 m correlate significantly with the 100 kyr−1 eccentricity cycle, the ∼20 m periods with the obliquity cycle, and the 15 to 11-m periods with both precession cycles. Wavelet transformation and decomposition of composite periodic functions were used to obtain the position of the cycle peaks in the profile. Cross-correlation with orbital cycles (La2004) dates the Baden-Sooss core between −14.379 ± 1 and −14.142 my ± 9 kyr.

Frictional strength of cuttings and core from SAFOD drillhole phases 1 and 2

We investigated the frictional properties of drill cuttings and core obtained from 1.85–3.1 km true vertical depth in the SAFOD scientific borehole in central California. Triaxial frictional sliding experiments were conducted on samples from primary lithologic units and significant shear zones, including the inferred active trace of the San Andreas fault. The samples were deformed at room temperature under constant effective normal stresses of 10, 40, and 80 MPa with axial shortening rates of 0.01–1.0 μm s−1. The weakest samples were from shale, claystone, and siltstone units with friction coefficient μ = 0.4–0.55. Stronger samples were from quartzo‐feldspathic rocks with μ ≥ 0.6. Materials tested from two shear zones at 2560 and 3067 m measured depth had μ = 0.4–0.55 and velocity strengthening behavior consistent with fault creep at depths <4 km. The coefficient of friction for bulk samples from the inferred trace of the San Andreas fault was ∼0.6.

Scientific investigation in deep wells for nuclear waste disposal studies at the Meuse/Haute Marne underground research laboratory, Northeastern France

Andra, the French National Radioactive Waste Management Agency, is constructing an underground test facility to study the feasibility of a radioactive waste disposal in the Jurassic-age Callovo-Oxfordian argillites. This paper describes the processes, the methods and results of a scientific characterization program carried out from the surface via deep boreholes with the aim to build a research facility for radioactive waste disposal. In particular this paper shows the evolution of the drilling programs and the borehole set up due to the refinement of the scientific objectives from 1994 to 2004. The pre-investigation phase on the Meuse/Haute-Marne site started in 1994. It consisted in drilling seven scientific boreholes. This phase, completed in 1996, led to the first regional geological cross-section showing the main geometrical characteristics of the host rock. Investigations on the laboratory site prior to the sinking of two shafts started in November 1999. The sinking of the shafts started in September 2000 with the auxiliary shaft completed in October 2004. The experimental gallery, at a depth of 445 m in the main shaft, was in operation by end 2004. During the construction of the laboratory, two major scientific programs were initiated to improve the existing knowledge of the regional hydrogeological characteristics and to accelerate the process of data acquisition on the shales. The aim of the 2003 hydrogeological drilling program was to determine, at regional scale, the properties of groundwater transport and to sample the water in the Oxfordian and Dogger limestones. The 2003–2004 programs consisted in drilling nine deep boreholes, four of which were slanted, to achieve an accurate definition of the structural features.

Stress and borehole stability

ABSTRACT Borehole and wellbore stability is a fundamental issue for natural resources production (hydrocarbons, water) as well as for subsurface storage and for scientific boreholes. This issue involves the in situ state of stress (often poorly known), rock mechanics behaviour, and drilling procedures. Many coupled physical mechanisms play a role: in situ stresses and pressures, variations in pore pressures and temperatures, and non-equilibrated physico-chemical processes arising in sensitive (swelling) shales. Useful results given by simple analysis tools are reviewed, along with case histories where mechanisms are considered and successful remedies proposed and implemented.

Evolution diagénétique dans la craie pélagique dolomitisée du crétacé supérieur du bassin de Paris (région de Provins, France)

Two deep scientific boreholes, named Poigny 701 and Sainte-Colombe 702, located in the Paris Basin near Provins (Seine-et-Marne, France), recovered a complete Upper Cretaceous chalk succession. A correlation between the boreholes lithostratigraphy, reflexion seismic profiles and diagenetic patterns shows that major velocity variations measured in the seismic reflection profiles correspond to dolomitized chalk intervals. Dolomitisations occurred during early and burial diagenesis. The understanding of these complex diagenetic events has an important economic consequence on the static correction of the chalk formation in the Paris Basin. Optimisation of petroleum prospecting below chalk cover is thus possible. The chalk series of the 701 and 702 boreholes range from the Cenomanian to the upper Campanian. In this succession, micrite has a primarily biogenic origin; it consists of pelagic organisms, indicative of warm seawater with values around 25°C. Several hiatuses occur in the 701 borehole. These hiatuses indicate the existence of particular hydrodynamic conditions. Deep-water channels were locally recognised in the Paris Basin as in Normandy and Picardy. In the 702 borehole, massive dolomitisation affected the upper meters of sediment below the sea floor. This early phase of massive dolomitisation was induced by slow circulation of a magnesium-rich seawater mass, along the seawater/sediment interface. Thus, this area was a zone of intense marine circulation between the North-West infralittoral and the South-West bathyal domains, across the London-Paris Basin. Magnesium-rich seawater had as origin the recrystallisation of the biogenic peri-plateform carbonates. During both dolomitisation and dissolution of the calcite matrix of the massive dolomite, the calcite had cemented chalk around the massive dolomite body. After deposition, the chalk series was progressively compacted and lithified by burial calcite cement. During the late Campanian-Maastrichtian, the burial compaction of the chalk and thermal gradient reached their maximum, as compressive stresses from the Pyrenean orogenesis affected the Paris Basin. At this time, compaction of the massive dolomite induced the expulsion of magnesian-rich fluids into the underlying already compacted chalk series. In 702 borehole, a diffuse dolomitisation then affected strongly underlying the chalk series. In 701 borehole, this diffuse dolomitisation affected slightly the lower half of the chalk series. Laterally, dolomitisation decreases gradually and affected a only smaller thickness of the chalk series, disappearing laterally. During the progressive emergence of the Paris Basin, from the Paleogene to the Quaternary, the chalk series were partially invaded by continental fresh water. Thus partial dedolomitisation affected the massive dolomite, whereas total dedolomitisation affected only the upper first meters of it.

Aptian source rocks in some South African Cretaceous basins

Abstract Source rocks of the Aptian play an important role in petroleum systems operating off the west coast of Africa. Further south, Aptian source rocks are a proven component of a petroleum system described in the Bredasdorp Basin, a sub-basin of the greater Outeniqua Basin. It has been shown that these Aptian source rocks have supplied oil to the Cretaceous sands of the producing Oribi oilfield and its satellite fields. Aptian source rocks have also been intersected by numerous exploration and scientific boreholes in other sub-basins of the greater Outeniqua Basin, and off the west coast of South Africa in the Orange Basin. Data from these wells suggest the regional development of good-quality Aptian source rocks in the deeper parts of the Orange and Southern Outeniqua basins, possibly on a basin-wide scale. In the Orange Basin, no Aptian source rocks were intersected in the boreholes drilled in the northern part of the South African sector. However, further north, in the Namibian sector of the basin, over 100 m of good-quality Aptian source rock was intersected in the Kudu wells. To the south, at about 31°S, organic carbon rich intervals a few tens of metres thick have been intersected in a number of boreholes. Here, the kerogen is largely gas-prone, with some thin intervals of shale capable of producing oil. Intersections of the Aptian in boreholes south of 32°S demonstrate how the source quality can be expected to improve from a proximal to more distal position. A thin, high total organic carbon (TOC), gas-prone interval in the most proximal position improves in quality and thickness to become up to 140 m thick, capable of generating wet gas and small amounts of oil. The Deep Sea Drilling Project (DPDS) 361 borehole, located to the southwest in the Cape Basin, intersected layered dark grey and black anoxic Aptian shales with very high TOC, with alternating marine and terrigenous influence. The hydrogen indices of over 500 mg HC/g TOC from some layers are clearly indicative of oil potential. There is thus a strong case for the regional development of a good-quality source rock within the Early Aptian succession in the deeper parts of the Orange Basin. Burial history studies show that Aptian sediments should be in the oil window in large areas to the west of the basin depocentre. The Early Aptian source rock in the Bredasdorp Basin has been well described. Here it can be over 200 m thick and occurs over a large areal extent. The organic material is largely Type II with a Type I component. Early Aptian source rocks have also been intersected in the Pletmos Basin further to the east, where the organic-rich interval is over 80 m thick in places. There is strong seismic evidence that this source interval should be well developed in the greater Southern Outeniqua Basin to the south of these two inboard basins. Burial history studies in this large basin show that the Early Aptian interval is sufficiently mature over large areas to have generated and expelled oil.

Abundance and Viability of the Groundwater Microbial Communities from a Borehole in the Tono Uranium Deposit Area, Central Japan.

The abundance and viability of microorganisms in groundwater were studied using a scientific borehole in the Tono uranium deposit area, central Japan. Groundwater samples were collected from scientific borehole TH-6 at four depths; 104, 132 and 153 m in sedimentary rocks and 177 m in granite rock using autoclaved geochemical water samplers. The total cell count using epifluorescence microscopy was of the order of 105 to 106 cells ml-1, showing non-systematic change in microbial parameters with depth. Viability estimated from cell membrane integrity ranged from 22.3% to almost 100%, and showed a tendency to increase with depth. In contrast, viability determined using both activities of the electron transport system (ETS) and esterase showed inconsistent depth-profiles. The ETS-active cell count corresponded to 0.57-24.7% of the total count; the minimum ETS-count was found at 153 m, just above the sediment-granite unconformity where the minimum redox potential was estimated. The esterase-active cell count corresponded to 0.4-10.3% of the total, with a maximum at 132 m, the lignite-derived organic-rich layer, and a minimum was found at 153 m, as seen for the ETS-active counts. These inconsistent profiles suggest a difference in microbial viability and activity between each depth. In addition, depth-specific microflorae may develop in response to the availability of various electron acceptors and donors in the subsurface.

Seismic results at Kola and KTB deep scientific boreholes: velocities, reflections, fluids, and crustal composition

Seismic studies from the Kola and KTB scientific boreholes are used to determine the origins of crustal seismic reflections and to study the effect of fluids, which are encountered through the entire depth range, on seismic wave propagation. Crustal seismic reflections are caused by compositional layering, shear zones, anisotropy and fluid-filled faults and fractures and all of these factors may contribute to one reflection group. Full wavefield recording and analysis, including S-waves and converted waves, may be used to distinguish the mixed origins of reflections. Sonic log velocities are systematically lower than VSP interval velocities because of the effect of drilling damage immediately around the borehole. A striking feature of both boreholes is the presence of brines in fractures and micro-cracks to TDs of 9 and 12 km yet the upper crust generally remains resistive, presumably due to low connectivity. Fluids enhance reflectivity in some zones and lower P-wave seismic velocity by about 0.2 km/s in the upper crust. Thus estimates of crustal composition based on seismic velocity are too felsic so that average upper crustal composition is more like felsic tonalite than granodiorite. In the Kola borehole, brines coexist at 12 km depth and 190°C; in the KTB borehole at 9 km depth, brines coexist with country rock at 265°C, and the depth at which brines disappear is unknown so that presence and effect of fluids in the deep crust should be re-evaluated.

Comparison of surface-, aquifer- and pore-waters from a Mesozoic basin and its underlying Palaeozoic basement, southeast France: chemical evolution of waters and relationships between aquifers

As part of the Deep Geology of France (GPF) `Ardèche' project, water samples were collected along the Ardèche margin of the Massif Central crystalline shield. Water is contained within various rocks, including Early Jurassic carbonate, Triassic sandstone and evaporite, Permian continental deposits, and the crystalline basement. Three sample types were studied: surface water from streams and springs, formation water sampled from fractured zones (aquifers) at various depths, and pore water from rocks cored in the `Morte-Mérie' scientific borehole, drilled to a depth of 980 m in 1993 as part of the `Ardèche' project. Three main water-chemistry signatures are associated with the three major geological groups: (1) HCO 3–Ca, Mg water occurs in Early Jurassic carbonate and Late Triassic sandstone with dolomitic cement; (2) Ca–SO 4 water occurs in Triassic sulphated shale and sandstone; (3) Na–Cl water occurs in Permian-sandstone and crystalline-basement. The samples enable a comparison between waters that flow on the surface, those that have been in the underlying geological formations for a short time, and those that have resided in closed pores of the host rock for a long time. Saturation indices computed for surface and formation waters show that dissolution reactions progressively increase the concentration of elements until saturation is reached. Surface waters were saturated for quartz and carbonate, and formation waters for quartz, carbonate, gypsum, muscovite and K-feldspar. Saline water (12 g/l TDS) from the Permian rocks is controlled by a mineralogical assemblage of quartz, calcite, anhydrite, muscovite, K-feldspar and albite, at temperatures 80–90°C. Although equilibrium was reached for the formation waters, concentrations in the pore waters reflect a general enrichment as compared to free formation waters from the fractured zones of the corresponding formation, which suggests introduction of mobile elements in the solution. Only in Permian rocks was a stable equilibrium observed since pore waters show no enrichment as compared to formation waters. No water mixing was observed, except along faults that form pathways for the fluid between formations. Each geological group generally behaves as a closed system, except the Permian rocks that seem to be (or were) exposed to circulation from the crystalline basement.

Shear-wave anisotropy and the stress field from borehole recordings at 2.5 km depth at Cajon Pass

53 local earthquakes recorded at 2.5 km depth in the Cajon Pass scientific borehole are analysed for shear-wave splitting. The time delays between the split shear waves can be positively identified for 32 of the events. Modelling these observations of polarizations and time delays using genetic algorithms suggests that the anisotropic structure near Cajon Pass has orthorhombic symmetry. The polarization of the shear waves and the inferred strike of the stress-aligned fluid-filled intergranular microcracks and pores suggests that the maximum horizontal compressional stress direction is approximately N13°W. This is consistent with previous results from earthquake source mechanisms and the right-lateral strike-slip motion on the nearby San Andreas Fault, but not with stresses measured within the uppermost 3 km of the borehole. This study suggests that the San Andreas Fault is driven by deeper tectonic stresses and the present understanding of a weak and frictionless San Andreas Fault may need to be modified. The active secondary faulting and folding close to the fault are probably driven by the relatively shallow stress as measured in the 3.5 km deep borehole.

Comparison of hydrogeochemical logging of drilling fluid during coring with the results from geophysical logging and hydraulic testing Example of the Morte-Mérie scientific borehole, Ardèche-France, Deep Geology of France Programme

A 980-m-deep well was cored on the Ardèche border of the Southeastern basin of France as part of the Deep Geology of France (GPF) programme. Hydrogeochemical logging was carried out during drilling, which involved the monitoring of physico-chemical parameters (pH, Eh, temperature and conductivity), and chemical parameters (concentrations of He, Rn, CO 2, CH 4, O 2 Ca, Cl and SiO 2) of the drilling fluid permanently circulating in the well. This logging programme was complemented by geophysical logging and two hydraulic tests. The combination of these measurements enabled identification of a transmissive interval due to fractures in the Jurassic carbonates, and of fluid inflow both at the base of the porous and slightly permeable Triassic sandstones and from an open fracture in the Permian conglomerates. These intervals are marked by changes in the drilling-fluid chemistry, such as an increase in chemical species content, or a drop in pH. The degree of modification depends on the natural permeability of the fractures and the salinity of the fluids. The porous and permeable intervals are also marked by He anomalies, which act as a tracer for these zones. Comparison between the geophysical and hydrogeochemical logs reveals that the latter provide information on the liquid phase, whether the fractures are productive or not, whereas the geophysical logs are more directly related to the solid phase.

The Ardèche palaeomargin of the South-East Basin of France: Mesozoic evolution of a part of the Tethyan continental margin (Géologie Profonde de la France programme)

This paper incorporates the results of seismic and gravity investigations, borehole data and studies of sedimentology, biostratigraphy, sequence stratigraphy and structural geology, carried out in the framework of the Deep Geology of France programme. The main purpose of the paper is to define the structure and to reconstruct the evolution of the Ardèche palaeomargin of the South-East Basin of France from the Stephanian to the end of the Mesozoic. The acquisition and processing of three seismic reflection profiles, the drilling of two scientific boreholes (BA1 and MM1), and subsequent structural modelling, with the use of cross-section balancing techniques applied to both the depth-migrated seismic and geological sections, provide new geometrical constraints which lead to the following hypothesis of the geodynamic evolution of the margin: 1. (1) Late Palaeozoic extension with the development of high-angle basement faults, asymmetrical Permian half-grabens, (pre-extension) sedimentary cover. 2. (2) During the Middle Triassic, a pre-rift stage with the creation of small fault blocks and, in the Late Triassic, the formation of an E-W flexure. 3. (3) End-Lias (Early Jurassic) to Middle Jurassic formation of a major detachment in the Carboniferous coal measures, leading to complete decoupling of the Mesozoic sedimentary cover from the basement. This episode of extension, coeval with the opening of Tethys, can be related to a phase of episodic rifting between the late Sinemurian and the late Bathonian (i.e. during 30 Ma). 4. (4) Late Jurassic to Early Cretaceous thermal subsidence, with regional downwarping of the basement towards the east. 5. (5) After the Late Cretaceous (the period of maximum burial), Pyrenean and Alpine deformation in the south and east, causing progressive uplift of the Ardeche palaeomargin, as much as two kilometres of Mesozoic deposits subsequently being eroded from the section during the Tertiary.

Palaeo-stresses inferred from macro- and microfractures in the Balazuc-1 borehole (GPF programme). Contribution to the tectonic evolution of the Cévennes border of the SE Basin of France

The Scientific borehole Balazuc-1 was drilled on the Ardèche margin of the SE Basin of France for the purpose of studying fluid-rock interactions in stratiform reservoirs in a tectonically controlled area. A structural study was made of oriented cores from the drill hole between the depths of 1218 and 1729.5 m; the various types of fracture that could have been used during fluid transfers have been described and analysed. Four major faults and 335 minor faults and tension gashes mineralized in carbonates, anhydrite and sulphides were intersected. At the bottom of the hole, one of the major faults brings into contact rocks of Triassic age and a formation attributed to the Upper Palaeozoic; the other faults have caused thinning of certain units of the Lower-Middle Triassic. Micro-structural study of the fault planes has enabled characterization of the tectonic episodes by which the Triassic and Lower Liassic sedimentary rocks were affected. These episodes, discussed in the context of the Ligurian Tethys, were: (1) E-W synsedimentary extension during the Early-Middle Triassic that caused tilting to the southeast of the Upper Palaeozoic and Lower-Middle Triassic successions; this continued into the Late Triassic. (2) An episode of N-S extension during the early Lias, characterized by hydroplastic-type faults, probably the result of a local variation of the stress-field in a trans-tensional regime. (3) The major period of activity on the Uzer fault, with downthrow of more than 1000 m, that continued from the Lotharingian (Late Sinemurian) to the Early Bathonian. It was not possible to calculate the stress tensor for this period, because no secondary fault system developed in the lower block traversed by the borehole. Brittle deformation at this time seems to have been concentrated on the major structures., (4) Approximately N-S extension, possibly end-Jurassic to Earliest Cretaceous, during which the tension gashes, mineralized in carbonates, and pyrite were formed in the Rhaetian and Hettangian deposits. (5) Three compressive and/or strike-slip episodes attributed to the Cenozoic Pyrenean and Alpine phases of deformation. The Oligocene extensional phase was not recognized in the borehole.

In situ stress measurements to 3.5km depth in the Cajon Pass scientific borehole: implications for the mechanics of crustal faulting : Zoback, M D; Healy, J H J Geophys ResV97, NB4, April 1992, P5039–5057

In situ stress measurements to 3.5km depth in the Cajon Pass scientific borehole: implications for the mechanics of crustal faulting : Zoback, M D; Healy, J H J Geophys ResV97, NB4, April 1992, P5039–5057

Deep Holes Yielding Geoscience Surprises

Deep Holes Yielding Geoscience Surprises

Fracture related reflectors in basement rock from vertical seismic profiling at Cajon Pass

Multi‐azimuth VSP data acquired between 500 and 2000 meters deep in crystalline rock penetrated by the Cajon Pass deep scientific borehole show several reflectors intersecting the hole. The reflectors correlate with VSP amplitude data, drilling and cutting logs, and various geophysical logs. Most appear to be related to fracture zones or lithologic transitions accompaanied by fracturing, mineral alteration and fluid circulation. Amplitude systematics from two azimuthally different offsets are consistent with the presence of a high angle fault in the neighborhood of the well.