Open Journal of Geology, 2014, 4, 18-23 Published Online January 2014 (http://www.scirp.org/journal/ojg) http://dx.doi.org/10.4236/ojg.2014.41002 Geophysical Data Bearing on Hydrocarbon Traps and Resource Potential of the Galisteo Basin, New Mexico Richard A. Ashu1, Michael S. Petronis2 1Harold Hamm School of Geology and Geological Engineering, University of North Dakota, Grand Forks, USA 2Department of Natural Resources, New Mexico Highlands University, Las Vegas, USA Email: [email protected] Received December 4, 2013; revised January 3, 2014; accepted January 11, 2014 Copyright © 2014 Richard A. Ashu, Michael S. Petronis. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. In accordance of the Creative Commons Attribution License all Copyrights © 2014 are reserved for SCIRP and the owner of the intellectual property Richard A. Ashu, Michael S. Petronis. All Copyright © 2014 are guarded by law and by SCIRP as a guardian. ABSTRACT Land-based gravity and magnetic data were obtained at 56 gravity stations and 250 magnetic stations through- out the northern-central part of the Galisteo Basin south of Santa Fe, NM. The study area extends south from the village of Eldorado to the foothills of the Ortiz Mountains and east from I-25 to Highway 285, an area of ap- proximately 2200 square kilometers. These data offered an approach to integrating surface geological mapping and subsurface geophysical surveys to characterize the distribution, depth, and potential hydrocarbon trap geo- metries in parts of the Galisteo Basin using the Talwani geophysical modeling program. Both sets of potential field data were integrated into ArcGIS and Surfer to generate extrapolated surfaces and derivative maps, which allowed for the characterization of the subsurface geology along with specific profiles across the mapped area. These data revealed several anomalies within the central portion of the basin that are interpreted as likely hy- drocarbon trap-structures. KEYWORDS Galisteo Basin; Hydrocarbon Traps; Gravity Survey; Magnetic Survey; Talwani 1. Introduction subject of debate and is not addressed in this research project. The evaluation of trapping mechanisms is a fundamental Potential field data were obtained using a gravimeter step in the analysis of prospective sites for resource as- sessments or successful oil and gas exploration. A trap and a proton magnetometer to constrain the basin archi- may be defined as a geometric configuration of rocks tecture. Merging field mapping information and potential suitable for containing hydrocarbons and sealed by a field data from gravity and magnetic surveys provided a relatively impermeable layer through which hydrocar- means to assess the petroleum potential of the Galisteo- bons will not migrate. The present work tested the hypo- Basin, and characterize the numerous igneous intrusions thesis that igneous intrusions during a late Tertiary mag- of various geometries that intrude the basin. The data matic event and earlier fold and fault of the country rock were modeled using the Talwani-forward modeling pro- contributed to the formation of sizable petroleum traps in gram [1]. Models were created along with different pro- the Galisteo Basin (Figure 1). In order to test this hy- files that characterize the deep basin geometry, likely pothesis, a geological mapping and geophysical survey- locations of structural traps, the relative positions of the ing (gravity and magnetic) campaign was conducted to basin sediments, and intrusions at depth. identify the geometry, distribution, and depth of the ig- The Galisteo Basin presents exploration challenges as- neous intrusions and other structures that likely serve as sociated with its complex structural architecture and an petroleum traps in some locations in the Galisteo Basin. overall poor understanding of its hydrocarbon systems. The Whether these traps are filled with hydrocarbons is a geology includes a wide variety of rock types including OPEN ACCESS OJG R. A. ASHU, M. S. PETRONIS 19 where accessible, from 56 gravity stations and 250 mag- netic stations within the northern-central part of the Ga- listeo Basin south of Santa Fe, NM. The study area ex- tends south from the village of Eldorado to the foothills of the Ortiz Mountains and east from I-25 to Highway 285, an area of approximately 2200 km2 (Figure 2). These new measurements were along public and private roads and were accessed mainly by car and foot. The gravity-magnetic measurements were established in re- gional traverses and combined with information from geologic mapping, core and log data, and seismic depth picks [4], to characterize the subsurface. Accurate loca- tion of survey stations were achieved using Garmin GPSMAP 60CSx equipment while absolute elevations were collected by means of a Brunton Nomad V2 digital compass. 2.1. Data Reduction The gravity data were corrected for instrumental drift, Figure 1. Boundary of the study area with regional struc- tides, latitude, and elevation. The final product of all cor- tures, volcanic fields, mountain ranges, and sedimentary rections and data reduction is a complete Bouguer ano- basins of north-central New Mexico (modified from [3]). maly map (Figure 3). The Bouguer anomalies correlate with lateral variation in density of the upper crust and are alluvium, sandstone, shale, siltstone, conglomerate, and determined by taking the difference between the ob- limestone of various geologic ages [2]. Prominent igne- served measurements and the theoretically calculated ous intrusions in this area are the Ortiz Mountains and value (GRS67) for that location and interpreted in terms Cerrillos Hills as well as numerous local volcanic struc- of subsurface properties. tures such as dikes, laccoliths, and other igneous geome- Magnetic data collected with the Geometrics 856AX tries throughout the Galisteo Basin. proton magnetometer were corrected for diurnal varia- The results from this research project provided a gen- tions and the regional field in much the same way as the eral insight into the petroleum exploration of other con- gravity data for drift and tidal effects. Having applied tinental rift zones, rifted margins and sedimentary basins these corrections, all remaining variations in magnetic in North Central region of the United States such as Che- field can be attributed to spatial fluctuations in the in- rokee Basin in the state of Wyoming and the Anadarko duced magnetic properties of the subsurface rocks re- Basin in the western part of the state of Oklahoma which ferred to as magnetic anomalies. Similarly, the final out- experienced similar geologic events. puts is imported into ArcGIS to produce a magnetic ano- maly map (Figure 4(A)). Geologic Setting The Galisteo Basin is one of a series of structural depres- 2.2. Model Construction sions composing the Southern Espanola Basin and part of Analysis of gravity and magnetic data requires the appli- the Rio Grande Rift, which formed during crustal exten- cation of mathematical methods to investigate the depth sion from late Oligocene (33.7 Ma to 23.8 Ma ) to and shape of the anomalies. The techniques employed in present. The Rio Grande Rift is a general north-south this research include gravity and magnetic anomaly sepa- alignment of structurally complex basins extending from ration and depth estimation. Anomaly separation applies southern Wyoming to Mexico. The Espanola basin is lo- enhancement techniques to isolate various long wave- cated in a major right step in the Rio Grande Rift system length components and provide a means to assess the re- with a general form of a west-dipping half graben. It ad- lationship between the regional, local, shallow, and deep joins the east-dipping San Luis basin on the northeast and sources. One such technique is the first derivative that the transitional Santo Domingo and east-dipping Albu- tends to increase the perceptibility of small amplitude querque Basin in the southwest. anomalies which are easily obscured by the regional 2. Methods and Data gravity field [5]. The first and second vertical derivative methods both Land-based gravity and magnetic data were obtained enhance near-surface anomalies at the expense of deeper OPEN ACCESS OJG 20 R. A. ASHU, M. S. PETRONIS Figure 2. Gravity and magnetic measurement stations established in the Galisteo Basin. anomalies and are good at locating the edges of a body tion of the different units within the study area had been with subsequent modeling to determine the depth, density, defined [3]. susceptibility contrast and geometry of the anomalous sources. Depth estimation techniques were subsequently 3. Results and Interpretations used to estimate the depths to the gravity or magnetic sources and were applied to gridded gravity data sets Seismic-reflection data support the northwest-trending (constructed in Surfer software) along three main profiles gradient of the gravity data shown on Figure 3. This across selected anomalies (Figure 4(B)). Within the study trend is attributed to the fact that a greater volume of area, deep gravity lows are associated commonly with lower density pre-rift sedimentary section lies on the accumulated sediments or volcanic tuffs which are less southwestern part of the basin, compared with the north. dense than the surrounding rocks. A density-depth func- In other words, the pre-rift sedimentary section is much OPEN ACCESS OJG R. A. ASHU, M. S. PETRONIS 21 Figure 3. Bouguer anomaly map of the study area with potential target areas for hydrocarbon traps (faults) within the study area. thinner in the north compared to the southern parts of the modeled as a single unit (Figure 5). Basin. A model of the North-South profile of the gravity data, constrained by the magnetic and related information 3.2. Interpreting Elements of the Basin Floor is presented as Figure 5(A), the Seton village profile as There are only a few areas of high magnetic signatures Figure 5(B), and the Eldorado profile as Figure 5(C).