REPORT COVERING COMBINED GRAVIMETRIC AND MAGNETIC SURVEY CAYUGA COUKTY, NEW YORK FOR NORTHEASTERN OPERATING COMPANY AND EASTERN GAS AND OIL BY EXPLORATION SURVEYS INC. ++++++++++++++++++++++++++++++++++ REPORT COVERING COMBINED GRAVIMETRIC AND MAGNETIC SURVEY CAYUGA COUNTY, NEW YORK INTRODUCTION This report pertains to a combined gravity and magnetic survey conducted by Exploration Surveys Inc. for Northeastern Operating Company and Eastern Gas and Oil over a designated area comprising most of Cayuga County, New York. The purpose of this survey was to obtain gravirnetric and magnetic information which might prove helpful in locating buried structual conditions suitable for the entrapment of hydrocarbons. FIELD OPERATIONS Field work in the assigned area began on March 6, 1974 and was terminated on May 2, 1974. A total of 1,798 stations were surveyed, metered and assigned gravity values. Magnetic values were also obtained for 1,603 of these stations. Although very inclement weather conditions were encountered during most of the period of operations, time lost due to bad weather was made up insofar as possible and the resulting station production is considered quite satisfactory, for the existing conditions. PROGRAM PLANS This work was initially considered as the first stage of a potent- ially more extensive program covering Cayuga County and adjacent areas in New York. As actually carried out, the program involved an initial period of quite detailed work in northern Cayuga County, designed to experimentally compare the gravimetric and magnetic results to available subsurface geology and history of hydrocarbon production, followed by reconnaissance coverage of essentially all of central and southern Cayuga County, with minor extensions of control eastward into Onondaga County. 2 As originally planned, this program involved Only the qravimetric survey. Prior to the commencement of field operations, it was suggested that a coordinated magnetometer survey, to occupy the same stations as the graviinetric survey, would furnish additional geophysical information at a relatively small added cost. The magnetometer survey was therefore added, on an experimental basis, for the first month of field work. The results led to a recommen- dation and approval for continuation of the magnetic work through the remainder of the program. DISCUSSION OF GEOPHYSICAL METHODS Gravimetric The objective of the gravimetric method of exploration is to obtain precise measurements of the earth's gravitational field at a series of observation points or "gravity stations" and to present a map of the resulting gravimetric values, corrected to remove all varia- tions other than those resulting from lateral changes in the densities of the earth's crustal materials. When this is accomplished with acceptable precision, the resulting "Bouguer Anomaly Map" represents the summation of the gravity anomalies produced by all mass irregu- larities whereby the earth's crustal rocks depart from an ideal homogeneous spheroid. This total gravity pattern is usually dominated by broad regional anomalies, which express the combined effects of large mass contrasts within the basement coirplex, regional changes in sedimentary thickness and gradual changes in lithology which affect average sedimentary density. Superimposed on this regional gravity background are the various more localized anomalies produced by local geological features and thus having potential structural and/or stratiqraphic significance in oil and gas exploration. Magnetic The magnetometer observations record lateral variations in the earth's magnetic field and, when temporal and broad regional variations have been removed, the remaining anomalies relate primarily to lateral changes in the magnetic susceptibility of the crustal rocks. Since the susceptibilities of the igneous rocks are very large in comparison to those of the sedimentary rocks, the observed magnetic anomalies arise principally from intraÄbasement contrasts and from basement topography, with relatively minor effects from sedimentary rock sources 3 ANTICIPATED ANOMALY SOURCES Gravimetric Available density logs within the area surveyed indicate compara- tively high sedimentary rock densities for the entire section from the surface downward. The indicated densities appear to be princi- pally in the range from 2.60 to 2.70 gm/cc or higher, the principal exceptions being the porous sand sections, where densities appear to be as low as 2.40 to 2.50 gm/cc. It thus seems reasonable to assume that localized positive structural features may create minor positive density contrasts within the sedimentary rocks, through compaction of the sediments and possible thinning of sands over the structural crests. It further appears, however, that the dominant density contrasts to be anticipated in this geologic province are those between differentially uplifted or tilted intra-basement blocks of acidic versus basic crystalline rocks and those between basement and sedimentary rocks, corresponding to basement topography. If the foregoing density-contrast assumptions are correct, it is believed that primary exploration emphasis should be placed on those maximum gravimetric anomalies which are most probably asso- ciated with up-toÄtheÄbasin displacements of uplifted or tilted high-density basement blocks. This tentative recommendation is based on the following assumptions: (1) Such basement uplifts should tend, through structural draping and/or rejuvenation of basement movement, to create positive structure in the overlying sediments; (2) Such features should tend to survive as continuing emergent highÄenergy zones, conducive to clean sand- stone porosity, possible carbonate porosity and possible reef growth; (3) Additional sands might tend to thicken, and thus to furnish localized stratigraphic traps, on the immediate flanks of such features. 4 Magnetic Due to the relatively very small magnetic susceptibilities of the sedimentary rocks, essentially all observable magnetic anomalies would be expected to originate from intraÄbasement or basement topography sources. Comparing the qravimetric and magnetic results, both should be dominated by intra-basernent contrasts, with the gravimetric measurements more strongly influenced by sedimentary rock density contrasts and the magnetic measurements more strongly influenced by basement topography. LIMITATIONS OF CONTROL Gravimetric Within the initial experimental area, covering primarily Brutus, Cato, conquest, and Mentz Townships in Northern Cayuga County, control was obtained at ¬Ämile spaced stations along all available roads, thus forming closed loops of roughly one mile to two miles in maximum dimensions. At least this degree of control detail is considered essential for optimum utilization of the gravimetric and/or magnetic measurements in this geologic province. At its current stage, this work in the central and southwestern portions of Cayuga County has been carried to a point of roughly 3 to 4 mile ioop dimensions, thus representing about 50% of the control needed for optimum elevation. Likewise, work in the southeastern portion of the county has now progressed only to a stage of 5 to 6 mi1e or larger loop dimensions, thus representing 25% or less of the desirable final control. In order to most fully utilize the present control, the BOuguer Anomaly Map has been contoured in the assumed most realistic manner and all subsequent data reduction and interpretation have been based on these Bouguer Anomaly contours. It is thus important, in all uses of the existing data, to carefully check the actual distribution of present station control and to recognize the limitations which are thereby imposed and the degrees of modification which might appear if fully adequate detail were added. 5 Magnetic The magnetometer observations in connection with this survey were made with an instrument capable of measuring the vertical component of the magnetic field with a precision of approximately +- (plus minus)1.0 gamma. The survey area is, however, traversed by numerous power lines, railroads, highways, pipelines and other artificial installations which severely disturb magnetic readings. These sources of inter- ference prevented the acquisition of usable magnetic values at 195 of the 1,798 gravity stations, while the average "noise level" of possible distortion by unrecognized artificial effects at the remaining stations may be as great at +-25.0 gammas. For the foregoing reasons, it was considered unwise to attempt contouring of the vertical magnetic intensity values with a contour interval closer than 50 gammas. On this basis, the resulting magnetic mapping may be assumed to express with fair accuracy the dominant magnetic anomalies which arise from intra-basement sources, but can hardly be expected to adequately delineate the more subtle magnetic anomalies which may result from basement topography or from magnetic susceptibility contrasts within the sedimentary rocks. In summary, the magnetic control is subject to the same basic limititations as the gravity data, with the further loss of values at about 11% of the stations and the imposition of a high level of distortion from extraneous surface influences. PRESENTATION OF DATA Gravimetric The basic presentation of gravity data appears in the "Bouguer Anomaly Map", which shows station locations, station numbers and Bouguer Anomaly values, derived as discussed in preceding report sections. In addition, the "Elevation Map" shows station locations and numbers, with surface elevations. This map may be utilized for comparing other data with surface topography. 6 In order to minimize the regional gravity effects and to emphasize those local gravity anomalies considered to have primary potential geological significance, various forms of residual maps are pre- pared. For this survey, two grid residual maps have been prepared, based on computer analysis of values interpolated from the Bouguer Anomaly Map for computation points forming a rectangular grid at 4,000' spacing. The "RB Grid Residual Map" utilizes an effective scanning radius of 17,888' and the "R4 Grid Residual Map" utilizes an effective scanning radius of 8,944'. The R4 residual map would be expected to more strongly emphasize the relatively narrow "hi3bÄ e~.ncyLarioma1i es produced by mass variations within the sedimentary rock's or by basement topography, while the RB_residual mapis probably more strongly influenced by major basement block contrasts at greater depth within the basement ~cOrT45T~. Both residual maps are presented with a colorÄcoding technique termed "Geocolor", whereby the contour curvature is utilized for separating the entire map area into maximunf anomalies (colored yellow) and minimum anomalies (colored green). The positive anomaly axes and outlines from the re~~dLive maps have been compared by use of a transparent overlay work map and the results are presented in a final "Anomaly Index Map". The indicated positive anomalous trends are given numerical designations, progressing from north to south across the area, and individual maximum anomalies along each trend are given alphabetical designations, progressing from ~ to east. This map is used as a reference in Lhe ensuing repoxt section `IS%L211 OF_ANOMAL?~'. It has been prepared with the Elevation Map as a base, so that the distribution of station control for the respective anomalies can be evaluated readily. Magnetic In view of the limited station control and high "noise level" of the magnetic measurements, it is not considered justifiable to prepare a further residual treatment of the magn‡tic data. The same limitations preclude adequate profile preparation for calculation of depths to the sources of the observed magnetic anomalies. 1 The presentation of the magnetic data has therefore been limited to a `GeocOlOr" treatment of the "Vertical Magnetic Intensity Map", followed by utilization of the overlay work map, mentioned above, to directly compare the resulting magnetic anomalies with the gravimetric anomalies and thus to make them available for examination while preparing the "Anomaly Index Map" and the final discussion of anomalies. GEOPHYSICAL - GEOLOGICAL CORRELATIONS Regionally, subsurface data in and around the survey area indicate an essentially east-west strike, with dip and thickening of sediments southward. The geophysical data indicate anomaly trend lineations which tend toward NE-SW orientations, possibly expressing additional intra-basement influences. interestingly, however, the Oriskany outcrop also crosses the survey area in a distinct NE-SW alinement. The initial experimental detailing covers an area including the Blue Tail Rooster Field (shut in Trenton gas) in Cato Township. A reasonably correlative positive anomalous expression appears here 4n both gravity and magnetic data, believed to support the concept of positive basement influence persisting in the sediments, at least as shallow as this middle Ordovician level. The extensive West Auburn Field (active Medina-Queenston gas production) is assumed to represent purely stratigraphic accumulation. Top of Queenston subsea values, however, show very pronounced structural nosing , if not actual minor closure. The relationship of this feature, as well as the open-flow potential figures, to the positive geophysical anomalies in this vicinity leads to conjecture as to the possible effects of underlying structure on structure and/or lithology at the Medina-Queenston level. The last work done in this program was in the vicinity of the Memphis_Field (abandoned Trenton and Potsdam gas) in Camillus and Van Buren Townships, Onondaga County. This work was added in the hope of determining the gravity and/or magnetic signature of an area of Potsdam g ~s accumulation, particularly in view of one theory that Potsdam sand development might occur along the basinward flank of a basement uplift or scarp. The time allowed for this work was insufficient either for completing a tie back to the data in Cayuga County…iforextending the control far enough north and south to fully define the local anomalies. The data obtained do suggest some type of posxtive~gravity and magnetic expression; also, based on page 8 the magnetic data, the Potsdam wells (east end of field) could conceivably lie on the southeast flank of a basement ridge or scarp. Aside from the three field areas just described, there appears to be no concentration of subsurface control within the survey area adequate for correlation with the geophysical data. DISCUSSION OF ANOMALIES This trend establishes the apparent correlation of Anomaly (lB) with the Blue Tail Rooster (Trenton)Field, with suggested west- ward and eastward trend extensions at Anomalies (1A) and (lc), respectively. Moderate magnetic confirmation is inferred at Anomalies (lB) and (1C). Trend (2 Anomaly (2A) is considered as one of the strongest prospects developed by this survey. The magnetic data indicate a possible "dipole anomaly", reasonably correlative with the strong gravity maximum. Anomaly (2D) is the suggested but not fully controlled maximum which is correlative with the Memphis (Trenton and Potsdam) Field. Anomaly (2B) is ~ while Anomaly (2C) and the suggested trendÄlineation eastward to Anomaly (2D) are merely inferred, due to lack of control here. Trend (3 This trend includes three inferred, weak gravity maxima. In the event that the possible basement scarp trend discussed in the preceding report section actually exists, these anomalies would appear to be related to segments of that trend. Trend (4) Anomaly (4B) is a strong gravity maximum, with suggested weak magnetic confirmation, in very interesting relationship to the West Auburn (Medina Ä Queenston) Field. Anomaly (4A) is a weak possible westward trend extension, while the eastward extension through Anomalies (4c) and (4D) is highly conjectural, based primarily on the magnetic data. 9 Trend (5) Anomalies (52«) and (SB) are apparent strong gravity maxima. The trend is only contingently recommended, however, due to lack of magnetic confirmation or clear trend delineation. Trend (6) This is considered as a potentially very significant trend. Anomalies (62«) and (6B) are strong gravity maxima with moderate magnetic confirmation, while Anomaly (6C) is an exceptionally strong .rnagnetic maximum with rather vague gravity confirmation. Trend (7 This trend is also quite conjectural and is merely noted for contingent future attention. Anomalies (72«) and (7C) are inferred strong gravity maxima, but not adequately controlled, while Anomaly (7B) and trend continuity are merely inferred. Trend (8 Anomaly (SB), as here outlined, is probably the top ranking prospect developed by this survey. This is a strong and exten- sive gravity and magnetic maximum complex, with suggested partial separation into northwestern and southeastern segments, indicative of possible fault disturbance of the postulated major basement uplift. Anomaly (82«) is a moderately strong gravity maximum, without magnetic confirmation. Anomalies (SD) and (SE) are moderate gravity maxima and Anomaly (SC) is a weak gravity maximum. Magnetic expression and trend continuity, eastward from Anomaly (8B), are vague. Trend (9) This trend exhibits varying anomaly character and poorly defined continuity. Anomalies (9A) and (9C) are relatively weak gravity maxima with apparently good magnetic confirmation. Anomaly (9B) is a moderate gravity maximum and Anomaly (9D) a weak gravity maximum, both lacking clear magnetic confirmation. page 10 Trend (10) Although trend continuity is vague, the four individual ano- malies which comprise this trend are implied strong gravity maxima, so far as present control allows an evaluation. Implied magnetic confirmation is strong at Anomaly (10D), moderate at Anomaly (l0B) and weak at Anomalies (10A) and (10C). Control is inadequate, particularly along the eastern half of this trend. Summary Top ranked prospects, at this stage of the survey, are Anomalies (8B) and (2A). Strongest magnetic confirmation is inferred for Anomalies (8B), (6c), (9c) and (10D). Complete review and reÄevaluation of these data should be carried out when and if additional geological and/or geophysical in- formation for the survey area become available. Dallas, Texas March 31, 1975 EXPLORATION SURVEYS INC. by __________________________ M. P. Jones