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Papers and Case Histories

Structure mapping at Trap Spring Oilfield, Nevada, using controlled-source magnetotellurics
[Abstract]  [Paper PDF:2087k]

The seismic reflection method has been a highly successful tool in oil and gas exploration for half a century, and it presently accounts for about 98% of all geophysical expenditures world-wide. However, the relatively high cost of seismic exploration and its limitations in certain geologic environments are continuing problems. Some help has been provided by the magnetotelluric (MT) sounding technique, but the cost of MT is also quite high due to the low natural signal strengths being measured.

The controlled-source audio-frequency magnetotellurics (CSAMT) technique is a shallower-penetrating variation of MT which uses an artificial signal source. This permits faster and more economical data acquisition. CSAMT has a penetration of about 2 km in typical petroliferous environments. CSAMT does not replace seismic but functions in three specific roles: (1) as a reconnaissance tool to help focus seismic coverage, or to help avoid 'no-record' zones; (2) to assist in static corrections and in interactive seismic interpretation; (3) as a primary tool in certain environments (volcanics, complex thrust areas) where seismic data acquisition is limited.

An example of the application of CSAMT to structure mapping comes from data taken over Trap Spring Field, located in the frontier Great Basin of the western United States. The field produces oil from fractured volcanics at the edge of a major graben fault. The CSAMT data delineate the major subsurface faulting and stratigraphic relationships in the area. The resolution of the CSAMT survey is significantly better than previously obtained induced polarization (IP) data. Detailed comparisons with electric log, drill hole, and air-photo data show an excellent correlation between the CSAMT features and known geology. The work suggests that CSAMT could be used in this area for reconnaissance mapping to develop seismic prospects, at approximately one sixth the cost of seismic.

Case Histories of an Electromagnetic Method for Petroleum Exploration
[Contents]  [PDF_1:3390k]  [PDF_2:6355k]  [PDF_3:4847k]  [PDF_4:3785k]
[PDF_5:4929k]  [PDF_6:5991k]  [PDF_7:1805k]  [PDF_8:5540k]  [PDF_9: 92k]

The results of a five-year feasibility study of the utility of electrical measurements in petroleum exploration are very encouraging. Electrical anomalies were measured over 66% of the 29 oil and gas fields involved in the study. The fact that these fields represent both stratigraphic and structural traps at a wide range of depths, with varying production characteristics, and in diverse geologic environments, provides a particularly favorable indication that this technique can be used as a reliable exploration tool, complementing existing seismic and subsurface geology programs.

Case histories of five field projects are presented in this volume in order to illustrate both the advantages and limitations of the method in the context of known geology. The field projects involve a variety of field characteristics. The results delineate the following general observations: 1) repeatable anomalies can be measured over oil and gas fields; 2) two types of anomalies are measured-an electrically conductive "deep anomaly," and a polarizable "shallow anomaly;" 3) the anomalies correlate relatively well in plan view with the limits of hydrocarbon production; 4) the anomalies are partly dependent on specific geologic characteristics of the stratigraphic sequence.

As is the case in seismic interpretations, data processing techniques are crucially important in extracting the maximum of information from electrical data. Proprietary data processing techniques have been developed for this purpose. Great care must be taken during interpretation to evaluate effects of pipelines, well casings, topography, and subsurface geology, since many of these effects may result in spurious anomalies unrelated to alteration due to hydrocarbons. The case histories demonstrate the necessity for this type of evaluation.

Induced Polarization Effects Associated With Hydrocarbon Accumulations
[Abstract]  [Paper PDF:920k]

The use of induced polarization (IP) methods in oil and gas exploration dates back to the 1930s, but the validity of anomalies has been difficult to establish. Although recent geochemical and downhole research has verified the source of IP anomalies in some geologic environments, the influence of cultural (anthropogenic) features on the electrical data remains a serious stumbling block to the acceptance of electrical methods in oil exploration. Spurious effects from power lines, pipelines, fences, and well casings can be misinterpreted as anomalies from hydrocarbon alteration or can mask true alteration anomalies.

The cultural problem is not insurmountable, however, and it is not valid to assume automatically that all IP anomalies measured over oil fields are the result of culture. A case study of the development of an oil field near Post, Texas, illustrates how proper survey design can be used to minimize and evaluate the effects of culture in the interpretation of IP survey data. Evaluation of before-and-after IP data sets and two-dimensional finite element modeling strongly support the interpretation that the observed IP anomaly results from hydrocarbon-induced alteration and not from well casing or other cultural effects. Furthermore, the interpreted extent of the IP anomaly as defined in 1982 agrees well with the productive limits of the field as it exists more than 12 years later.

Locating Abandoned Wells
[Abstract]  [Paper PDF:2149k]

Subsurface Technology, Inc. and their subcontractor, Zonge International Organization, Inc., were contracted by the Solution Mining Research Institute to conduct a Survey of Methods and Commercial Resources for Locating Abandoned Wells. The current volume is the result, presenting descriptions of methods that have been used, or have the potential for use, in detecting abandoned artificial penetrations. Included are descriptions of selected methods, explanations of the physical quantity being measured, discussions of procedures, cost estimates, and resources for procuring services to implement the methods.

The goal of this manual is to provide a resource to persons responsible for the safe operation of storage cavern facilities. No manual such as this can provide a prescriptive set of procedures to follow. Rather, the intent of this manual is to provide guidance for educated selection and supervision of service companies that provide the various methods.

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