THE AIM OF THIS PROJECT IS TO VERIFY THE ASSOCIATION BETWEEN NEAR-SURFACE DIAGENETIC MAGNETITE AND HYDROCARBONS AT DEPTH, LEADING, IF SUCCESSFUL, TO A RADICALLY DIFFERENT, MUCH MORE ECONOMIC HYDROCARBON EXPLORATION.
The theory that diagenetic magnetitie may form above certain hydrocarbon accumulations in sufficient concentrations to be detectable by high resolution aeromagnetic survey has been investigated. Aerogeophysical data sets over 2 known, and 1 prospective hydrocarbon field have been examined. An extensive library of innovative software has been developed to facilitate the selective enhancement and display of high frequency, low amplitude aeromagnetic anomalies. Surface magnetometer traverses were carried out to investigate potential anomalies seen in the aeromagnetic data and laboratory work was undertaken to identify magnetic material found in the cuttings of boreholes drilled in the 2 known oil fields. No hydrocarbon induced magnetic anomalies were detected and none of the oilfields investigated possessed a characteristic 'halo' spectrometric signature.
The airborne data was plagued with cultural noise and a satisfactory routine for either eliminating this or for extracting potentially useful anomalies has not been found.
Ground follow up traverses failed to substantiate several low amplitude aeromagnetic anomalies that did not appear to have a cultural source and, which due to their character, were considered prospective.
A further problem arosefrom gridding high density profile data from widely spaced lines. New schemes were developed for the analysis and display of fine detail in aeromagnetic surveys which effectively combined aerial and profile information.
No significant amounts of diagenetic magnetite were found in borehole samples from the Formby and Welton areas. Magnetic susceptibility measurements on cuttings from both productive and dry boreholes in these areas yielded occasional peak readings but no extended sections of elevated values were recorded. The principal magnetic component of the samples displaying increased susceptibility could be attributed to drilling contaminants. The preponderant natural magnetic material of the Formby samples was detrital, consisting of magnetite and ilmenite that particles of diagenetic magnetite in scarce, unidentified magnetic material from the Formby area would have no measurable magnetic effect. Identifiable natural magnetic material from the Welton area was exclusively detrital.
IT IS SUGGESTED THAT CHEMICAL ALTERATIONS IN THE NEAR-SURFACE SEDIMENTS, BROUGHT ABOUT THE MICROSEEPAGE OF HYDROCARBONS AT DEPTH WOULD RESULT IN THE FORMATION OF DIAGENETIC MAGNETITE WHICH SHOULD GIVE RISE TO LOW-AMPLITUDE, HIGH SPATIAL FREQUENCY MAGNETIC ANOMALIES.
THESE COULD BE DETECTED BY GROUND OR AIRBORNE HORIZONTAL GRADIOMETRY SURVEY.
A RESEARCH PROGRAMME IS UNDERTAKEN TO DETERMINE WHETHER SUCH A PHENOMENON OCCURS OVER KNOWN UK OIL AND GAS FIELDS AND IF SO, TO DEVELOP EXPLORATORY CRITERIA. VOLUME SUSCEPTIBILITY AND REMANENT MAGNETIZATION DETERMINATIONS WILL BE CONDUCTED ON STATISTICALLY MEANINGFUL CORE SAMPLES AND OIL WELL CUTTINGS FROM FORMBY DISTRICT. WHERE POSSIBLE, SUSCEPTIBILITY LOGS WILL BE RUN IN OILWELLS USING A GEOSINSTRUMENTS TH-3C PROBE AND USED IN PREFERENCE TO THE LAB. DETERMINATIONS.
MAGNETIC PHASES WILL BE IDENTIFIED BY ELECTRON MICROPROBE ANALYSIS AND XR DIFFRACTION AND THEIR GENESIS ASSESSED BY MORPHOLOGICAL EXAMINATION. ADVANCED COMPUTATIONAL AND IMAGE DATA PROCESSING TECHNIQUES WILL BE DEVELOPED BASED ON THE USE OF LONGITUDINAL HORIZONTAL DERIVATIVES REDUCTION TO POLE, DEVELOPMENT OF BAND-PASS FILTERS AND FOURIER DECOMPOSITION FOR BETTER NOISE SUPPRESSION.
COMPLEMENTARY HIGH QUALITY GROUND MAGNETIC DATA WILL BE ACQUIRED ON GRIDDED LINES ACROSS SEVERAL HYDROCARBON FIELDS IN THE AFOREMENTIONED DISTRICT, USING A DIGITAL GRADIOMETRIC PROTON MAGNETOMETER. AFTER PROCESSING ANOMALOUS SIGNATURES NOT REFERABLE TO KNOWN GEOLOGIE AND CULTURAL FEATURES WILL BE LINKED TO SHALLOW EPIGENETIC MAGNETIC MINERAL ENRICHMENT. IN CO-OPERATION WITH NERSC (CONTRACT EN3C001200-UK) REFINED QUANTITATIVE 3D THEORETICAL MODELLING WILL BE UNDERTAKEN TO RELATE MAGNETIC ANOMALIES TO THEIR SOURCES. TAKING INTO ACCOUNT THE INDUCED MAGNETIZATION IN THE EARTH'S FIELD, THE MAGNETIC SUSCEPTIBILITY MAPS WILL BE TRANSFORMED INTO APPARENT SUSCEPTIBILITY MAPS WHICH SHOULD APPROXIMATE THE TRUE DISTRIBUTION OF MAGNETITE. NEW NEAR-SURFACE ZONES OF EPIGENETIC MINERAL ENRICHMENT MAY THUS BE IDENTIFIED AND RANKED IN TERMS OF THEIR HYDROCARBON PROSPECTIVITY. RECOMMENDATIONS FOR TEST DRILLING WILL BE MADE.