ASEG Extended Abstracts - ASEG2007 - 19th Geophysical Conference, 2007

Data from 50 sonobuoys were recorded over the Capel and Faust Basins, 800 km to the east of mainland Australia in water depths 1500-2000 m, during a 2006 seismic survey for Geoscience Australia. These data were interpreted and forward modelled by ray-tracing to provide an estimate of P-wave velocities in the upper sedimentary section, and hence constrain estimates of sediment thickness. Also recorded were gravity and magnetic anomaly data which, in conjunction with the very high quality seismic reflection data, provided additional constraints upon the velocity models. Typical ranges in four model layers below water were: 1.9, 2.3-3.0, 3.6-4.7, 5-5.3 km/s. Gravity models based on these results were compared to features identified on depth converted seismic reflection lines and indicate that sediment thickness at densities approximating 2.3 t/m3 may reach 5 km in several localities.

In this paper, we propose a new interpretation method of gravity data by means of combining analytic signal and Euler deconvolution. Euler deconvolution is widely used to analyze potential data without any prior information. However, since Euler deconvolution tends to make so many spurious solutions, it is hard to pick up real solutions. Avoiding these ambiguous problems, we utilized a combined method between analytic signal and Euler deconvolution. Analytic signal can give us horizontal information of isolated anomalous bodies. We carried out Euler deconvolution not only gravity data but also analytic signal of gravity data. It gives us clearer solutions than using Euler deconvolution only. We verified the proposed method by synthetic data and applied it for microgravity data. We carried out gravity survey using Scintrex CG3, CG5, and ZLS Burris meter. The target area is a small urban area nearby coal mines. This area has some cases of subsidence problem due to cavities in limestone bedrock. We set up 10 profiles and measured every 4 m. And the some part of area was collected by scattered points because it is impossible to make a profile due to existence of buildings. We gathered totally about 1100 points. Low Bouguer anomalous zones coincided well with drill logs. We applied integrated interpretation method to microgravity data of limestone area by means of analytic signal and Euler deconvolution simultaneously. Results from the combined method showed indications of cavities.

Technical Area: Environmental (environmental and engineering geophysics)

Australian waters contain a number of under-explored offshore sedimentary basins, which have or could have significant hydrocarbon accumulations. Current exploration strategies are based primarily upon seismic data acquisition, with other data type acquisition, (e.g. geochemical sampling) requiring further survey cruises. This multiple survey strategy significantly increases exploration costs leading to tough decisions being made upon which surveys to perform when exploration budgets are tight. The attraction of using a single cruise to achieve multiple surveys is therefore of interest in reducing costs and increasing the impact of each exploration dollar.

New chemical-physical sensing devices offer potential for useful new technology to explore large areas of offshore basins to detect microseeps and provide molecular information that is indicative of fluid type. These sensors when mounted on current survey platforms could be run continuously in marine surveys to obtain profiles of hydrocarbons in water that can be mapped in a similar way to seismic, electromagnetic, and magnetic data.

This paper reviews currently available technologies and presents the collaborative research being conducted by CSIRO Divisions of Petroleum and Industrial Physics, Curtin University, and University of Western Australia. This research is focusing upon the two essential parts of chemical sensing systems, the nanochemical molecular binding element that is often a surface designed to be specific for certain classes of molecule, and a transduction element that provides a physical signal that binding has taken place. These elements will then be integrated into a robust marine deployable sensor system that can be used for hydrocarbon exploration.

Technical Area: General – Radar and remote sensing or Instrument forum

Lynch Mining holds numerous exploration licences on the Eyre Peninsula where there is very little outcrop although the average cover is only a few tens of metres. Geophysical methods are crucial to exploration in these circumstances. Early drilling of a large (7000 nT) ~2500-3000 m  350 m Magnetic anomaly, intersected east dipping (60°-80°) iron formation and associated forsterite-magnetite marbles and calcsilicates. A zone between 87 m and 125 m of BLDD06 assayed at 40% Fe with low (<900 ppm) phosphorus and ~ 3000 ppm manganese. From the size and area of the magnetic anomaly, a large iron ore resource was inferred. Initial magnetic modelling indicated an elongated broad vertically dipping body. Subsequent drilling, while intersecting significant (up to 70 m downhole) interpreted lens-like bodies, failed to intersect a thick uniform ironrich body commensurate with the initial magnetic modelling.

Magnetic property measurements (k >1.0 SI) suggest that the magnetic susceptibility value used in the initial modelling (0.5 SI) was too low by a factor of two. Moreover, the remanence is viscous and probably in the same direction as the induced magnetisation so the effective susceptibility may be >2.0 SI. Selfdemagnetisation constrains the magnetisation to be aligned along the bodies, deflected from the geomagnetic field direction, so that no matter how the bodies dip the anomaly will be symmetric. The causative bodies appear to be dispersed magnetite-rich lenticular pods giving a combined anomaly that simulates a uniform body.

The aim of this study is to verify that the low impedance and over-pressured petroleum systems (gas reservoirs) in the vicinity of high stresses are unproductive with the hydrocarbons. The study has been conducted in and around the Sawan Gas Field, located in Southern Pakistan. Rock Physics Parameters (Poison’s Ratio, Vp/Vs Ratio) are calculated in this area. The behaviour of these properties resembles to that for low impedance and over-pressured gas reservoir, which in this case is the Lower Goru Sands. Sequence stratigraphic studies are carried out to comprehend the depositional model of sand & shale. Conformity has been established between this model and the pattern achieved from detailed rock physics investigations, which further helped in the identification of the anomalous gas zones for the reservoir. Next, it needs to be confirmed, whether or not these anomalous zones can be productive? This goal is achieved by the following rheological studies.

• Finding the longitudinal and shear strain around the interpreted faults.

• Converting this strain into the stresses.

• Contouring the Stress and Strain values.

These contour maps give a complete picture of where the reservoir falls in high stresses zone? In these areas it is not productive. On the other hand, we can predict its productivity in the location of low stresses. These scenarios can be demonstrated by examples of some productive and failed wells in the field.

Technical Area: Petroleum Geophysics

The NMO/DMO/stack method is a traditional and wellknown method in the oil industry that needs an accurate macro velocity model to image the subsurface structures. Making such a macro velocity model is a time consuming process that is error prone. New introduced commonreflection-surface (CRS) stack is a data driven method which is independent of velocity information apart from the surface velocity. It comes from common reflection point (CRP) trajectory concept for finite offset. In NMO/DMO/stack, the stacked data are obtained from a summation over a curve along the offset coordinate but the principal of CRS stack is to sum along a surface of specular contributions from a segment of a reflector instead of reflection point. The summation over a segment of a coherent reflector drastically improves the signal/noise (S/N) ratio as the stacked data will show. An important aspect of the method is that the estimated parameters provide us with significant information on the subsurface structure. These are three new parameters called kinematic wavefield attributes; , RN and RNIP. The parameter  is the emergence angle of normal-ray which will be used later for a normal-ray map migration. RN is the curvature of the exploding reflector wavefield measured at the surface and RNIP is the curvature of normal-incidence-point wave which could be used later to yield information on the propagation velocity and inversion NIP tomography. Here we processed a synthetic data to derive zero offset or CRS stacked section with high S/N ratio and better continuity on the reflection events.

The Otway Basin CO2 sequestration pilot project aims to demonstrate that CO2 can be safely stored in a depleted gas field and that an appropriate monitoring strategy can be deployed to verify its containment. The advantage of injecting CO2 into a depleted gas field is having access to well-established infrastructure, pre-existing geophysical exploration data, and production wells. On the downside, the geological complexity of the Naylor gas field, which is relatively deep and of a small size (0.5km²), presents challenges for detailed geophysical and geological characterization and consequently makes the design of a geophysical monitoring program much more difficult. Uncertainty of the location of paleo and current gas-water-contact poses additional difficulty for positioning of the injection well. These factors call for further analysis of all available geophysical data.

One such task is the investigation and examination of the elastic properties changes at varying saturation and pressure in time-lapse and their effect on seismic response before and after CO2 injection at the existing Naylor-1 well (monitoring well). The result from modeling shows that the density is more sensitive than velocity. Consequently, model-based prediction suggests the changes in elastic properties and the effect in seismic response is very subtle. Attempt to relate the seismic attributes computed from pre-stack and post-stack 3D seismic and VSP to the changes in elastic properties will help further refine the geophysical analysis.

Using the same methodology, the modeling of the new injection well is compared with Naylor-1 model. The outcome will be discussed in this paper.

A VTEM airborne EM and magnetic survey was completed over an area in southwest Western Australia in 2004. The purpose of the program was to aid in the identification of additional base and precious mineralization as had previously been encountered in follow-up drilling to a Tempest airborne EM and magnetics survey.

As part of the assessment of the VTEM results, layered earth inversion and parametric modeling of the EM responses was undertaken. The outcomes of this work showed the two approaches were complimentary and appear to highlight different aspects of the mineralized system.

The marine Controlled-Source Electromagnetic (CSEM) method has proven itself an invaluable adjunct to the seismic method in petroleum exploration in the last few years. The problem is that it is not easily adapted to the Australian scene because most of Australia’s petroleum deposits are in relatively shallow waters where CSEM does not work very well. Depending on the target, depths up to 500 m can be considered shallow water for CSEM. CSEM surveys in shallow water can have problems with the airwave dominating the received CSEM signal and also with electromagnetic noise induced by ocean waves. Modelling studies suggest that it is feasible to measure CSEM magnetic field gradients as a supplement to E and B field measurements and use this extra information to remove environmental noise (wave effects and magnetotelluric noise), airwave and direct wave components, and other unwanted contributions to the measured E and B fields. This can improve the signal/noise ratio of the CSEM measurements, thereby producing more reliable interpretations, and extend the useful range of the method, which can reduce survey costs. We have been developing a magnetic gradiometer that can be used for marine CSEM surveys. This poster will present the results from preliminary field trials conducted off Sydney’s coast, in water depths up to 62 m, which show that the OceanMAG gradiometer was able to measure the magnetic field induced by ocean waves. Approaches for using magnetic gradient measurements as an adjunct to marine CSEM data will also be described.

Technical Area: Petroleum - Electromagnetics

There are numerous experimental works intending to find the dependence between porosity and resistivity of sediments of different kind and different age. However there is not enough application of these theoretical considerations to the interpretation of electromagnetic data and physical modeling of data. Mathematical modeling of petrophysical properties can be done using matrix (fractal) models. A fractal model containing nseries of spheres with corresponding radii and surrounded by a thin film of adsorbed water (DEL) has been used. This model is more suitable for real sedimentary rocks. Using the parameters of the fractal model several petrophysical parameters can be calculated namely: resistivity, total and effective porosity, permeability, diffusions coefficient CD applied to seismoelectric sounding, volume of matrix and thickness of adsorbed water which characterizes elecroosmosis polarizability and decay constant

Uncertainties in the quantitative interpretation of well log data are often unquantified and ignored. Uncertainties are always present to some degree and have many underlying causes such as quality of log data, lack of calibration data, choice of petrophysical model, choice of model parameters. Understanding the nature of reducing the uncertainties is important if the petrophysical results are to be used for reservoir characterisation, reservoir modelling or risk analysis.

Statistical petrophysical techniques provide a means to assess the sensitivity of the interpretation to model and model parameter choice and to include known or assumed uncertainties in the measured data. To reduce uncertainty additional information must be introduced such as multiple well data or rock physics constraints. Parallel interpretation of multiple wells can increase the signal to noise ratio of the interpretation and at least ensures greater consistency of interpretation between the wells. Rock physics provides a link between the standard petrophysical properties and any measured elastic properties of the formation and therefore can provide an additional constraint on the petrophysical interpretation. An extra advantage of integrating a rock physics model is that, since elastic properties can be estimated from seismic data, the petrophysical interpretation can be interpolated between wells more accurately using seismic data as a guide.

This paper highlights how statistical log analysis, parallel interpretation of multiple wells and the integration of rock physics constraints can reduce uncertainty in petrophysical interpretation in cases of bad hole and complex lithology.

Technical Area: Formation evaluation/Petrophysics/Borehole geophysics

(FEWSA member)

In the DF1-1 Gas Field in the Yinggehai Basin, South China Sea, the velocity-depth plot and velocity spectra show significant variations from the classic linear trend, exhibiting a universal reversal phenomenon. Velocity parameters derived from velocity spectral analyses of the seismic data and sonic logs indicate that the “Interval Velocity” universally reverses below 2100 m, corresponding to the starting depth of overpressure in the field. There is a distinct difference between the gas-bearing sandstones and the surrounding rocks in the shallow strata with depths <2100 m, however, such a difference becomes less apparent beyond 2100 m in the middle-deep strata.

In the shallow strata of the DF1-1 Gas Field, the gas-bearing sandstones can be effectively recognized prior to exploration drilling using the DHI (Direct Hydrocarbon Indicator) techniques. However, these DHI models developed for the shallow strata were found to be ineffective for the middle-deep strata for direct exploration target recognition due to the velocity reversal.

To effectively identify DHIs in the middle to deep depth strata under velocity inversion, we used an integrative approach to detect the “integrated hydrocarbon-indicators” and tested the applicability of “Differential Interformation Velocity Analysis (DIVA)” as a DHI in the DF1-1 Gas Field. The results indicate that the “DIVA” technique can be effectively used as a DHI in both the shallow and the middle-deep strata in the study area with the shallow strata characterized by Type I DIVA anomaly and the middle-deep strata characterized by the Type II DIVA anomaly.

Technical Area: PETROLEUM