Limited Borehole Data Research Articles

Detection and correlation of tephra-derived smectite-rich shear zones by analyzing glass melt inclusions in mineral grains

Shear zones rich in the clay mineral smectite are significant geological features because of the particular physicochemical properties of this mineral. However, predicting the distribution of such shear zones using limited borehole data is a challenge for geoscientists and geoengineers. Here we present detailed mineralogical and geochemical analyses of clay-rich shear zones (≤30cm thick) that are associated with bedding-parallel or bedding-oblique faults in drill cores of a siliceous mudstone (containing mainly opal-cristobalite/tridymite) from the Horonobe area, northern Hokkaido, Japan. The bedding-parallel shear zones are rich in smectite and some shear zones are characterized by melt inclusions hosted by plagioclase and quartz. The chemical compositions of melt inclusions in six shear zones are very similar, but distinct from those of melt inclusions in other shear zones. These six shear zones consistently occur in horizons ~350m shallower than a known tephra layer. These mineralogical characteristics and stratigraphic positions suggest that the six shear zones occur within the same tephra layer and that the smectite formed as a result of burial diagenesis of volcanic glass. Tephra-derived smectite-rich shear zones can be detected and correlated by analyzing melt inclusions preserved in mineral grains. The distribution of such shear zones can be predicted in a similar way to tephra layers.

Using geoarchaeological deposit modelling as a framework for archaeological evaluation and mitigation in alluvial environments

The pace of anthropogenic development on floodplains and adjacent valley floors is still increasing and in many countries this is accompanied by a requirement for heritage mitigation and management. The result is an increased demand for effective and efficient archaeological evaluation and mitigation strategies, which can only be achieved in alluvial environments through the application of geoarchaeological methods. This paper uses lidar data combined with deep geophysical survey (electrical resistivity), gouge coring and limited borehole data to derive a three dimensional geoarchaeological deposit model, which provided a vehicle for archaeological evaluation and mitigation. Significantly, the results of this deposit model are compared to the results from the subsequent archaeological evaluation trenching, a methodological next step that has not received sufficient attention within the (geo)archaeological literature. The deposit model is refined using radiocarbon dating and artefactual evidence derived from the archaeological evaluation trenching. The results demonstrate how geoarchaeological deposit modelling can be integrated with archaeological evaluation trenching and provides discussion of the importance and difficulties of integrating geoarchaeological sediment units (archives) with archaeological contextual excavation data, with conventional stratigraphic matrices.

Vesicle distribution in basalt lava flow units in the Mesozoic rift basins of northeast China and its application in gas reservoir prediction

The Mesozoic rift basins of northeast China are characterized by a significant proportion of basalt due to the progressive emplacement of basalt lava flows. The objective of this study was to construct vesicle distribution models of the basalt lava flow units, including conceptual geological models and thickness models, to understand the architecture of the basalt and the gas reservoir distribution. The conceptual geological models were constructed based on the characteristics of outcropping basalt lava flows, with supplemental seismic data used to extrapolate the lateral extents of large-scale basalt lava flows. The thickness models were constructed using data on the thicknesses of basalt lava flow units and vesicle zones. These data were obtained from 27 units in outcrop and 204 units interpreted from well logs in the basin. The conceptual geological models revealed that the shapes of the basalt lava flow units change from braided to tabular with increasing thickness and that their inner structures can be divided into three vesicle zones: the top vesicle zone, the massive core zone, and the base vesicle zone. The thickness models revealed that trends in the thickness of the top vesicle zone relative to the thickness of the basalt lava flow unit can be expressed using a piecewise function that can be separated into a linear function and a logarithmic function. Similarly, trends in the thickness of the massive core zone and the base vesicle zone relative to the basalt lava flow unit thickness can be expressed by a piecewise linear function. Vesicle distribution models provide an effective means of determining the proportion and distribution of vesicle zones in basalt with limited borehole data. We also constructed a reservoir model based on our vesicle distribution models, and this model revealed that suitable petroleum reservoirs are primarily located in the thinner braided lava flows.

NEW DEPTH MAPS OF THE MAIN KAROO BASIN, USED TO EXPLORE THE CAPE ISOSTATIC ANOMALY, SOUTH AFRICA

Here we present a comprehensive depth and thickness map of the main Karoo and Cape Basins using borehole and reflection seismic data. The depth to the Whitehill Formation, which is the focus of current shale gas interest within the Karoo, is also mapped. Change: The deepest part of the basin is in the south, along the northern boundary of the Cape Fold Belt (~4000 m in the southwest Karoo and ~5000 m in the southeast; ~5500 to 6000 m sediment thickness). The Whitehill Formation along this boundary reaches a depth of ~3000 m in the southwest and ~4000 m in the southeast. Limited borehole data in the southeastern Karoo show a broad deepening of the basin here compared to the southwestern Karoo. In the southeast near East London faulting has resulted in deepening of the basin close to the coast, with the Whitehill Formation deepening to over ~5000 km. Seismic and borehole data show that the Cape Supergroup pinches out below the Karoo Basin around Beaufort West and Graaff-Reinet in the southern Karoo (32.6°S for the Bokkeveld and 32.4°S for the Table Mountain Group). The Cape Supergroup reaches thicknesses of around 4 km in the south. The gravity effect of these sediments does not account for the Cape Isostatic Anomaly (CIA) in the southern part of the Karoo Basin near Willowmore and Steytlerville, i.e., an ~45 mGal Bouguer gravity low. A refraction seismic profile over the anomaly shows this region is associated with a large volume of low velocity/density shallow sediments (4.5 m/s2, 2500 kg/m3), as well as a low velocity/density anomaly associated with a normal fault and the Klein Winterhoek Thrust Fault (5.5 m/s2, 2650 kg/m3). These low density shallow sediments are explained by uplift of Karoo and Cape sediments of ~2 km or greater that is evident on Soekor reflection seismic data. This deformation has brought lower density shales (1800 to 2650 kg/m3) of the Ecca Group closer to the surface. These shallower features along with a deeper lower crust in this region (6.5 m/s2, 2900 kg/m3) are interpreted to account for the CIA.

Hydrogeological parameter estimation using geophysical data: a review of selected techniques

Subsurface environmental, engineering, and agricultural investigations often require characterization of hydraulic parameters. For example, groundwater flow modeling is often performed through an aquifer whose hydrological properties have been created using stochastic simulation techniques; these techniques use as input both hydraulic parameter point values and spatial correlation structure information. Conventional sampling or borehole techniques for measuring these parameters are costly, time-consuming, and invasive. Geophysical data can compliment direct characterization data by providing multi-dimensional and high resolution subsurface measurements in a minimally invasive manner. Several techniques have been developed in the preceding decade for using joint geophysical–hydrological data to characterize the subsurface; the purpose of this study is to review three methodologies that we have recently developed for use with geophysical–hydrological data to estimate hydrological parameters and their spatial correlation structures. The first two methodologies presented focus on producing high-resolution estimates of hydrological properties using densely sampled geophysical data and limited borehole data. Although we find that high-resolution geophysical data are useful for obtaining these estimates, in practice, geophysical profiles often sample only a small portion of the aquifer under investigation, and thus, the estimates obtained from geophysical data may not be sufficient to completely describe the hydraulic properties of the aquifer volume. The third and last section focuses on using high-resolution tomographic data together with limited borehole data to infer the spatial correlation structure of log-permeability, which can be used within stochastic simulation techniques to generate parameter estimates at unsampled locations. Our synthetic case studies suggest that collection of a few tomographic profiles and interpretation of these profiles together with limited wellbore data can yield hydrological point values and spatial correlation structure information that can be used to aid numerical aquifer model construction, calibration, and flow simulation. As this information is typically only obtainable from extensive hydrological sampling, use of geophysical methods may offer a more efficient and less invasive approach than traditional characterization campaigns.