Efraín Méndez-Hernández

A passive seismic survey over a gas field: Analysis of low-frequency anomalies

Passive seismic low-frequency from approximately 1–6 Hz data have been acquired at several locations around the world. Spectra calculated from these data, acquired over fields with known hydrocarbon accumulations, show common spectral anomalies. Verification of whether these anomalies are common to only a few, many, or all hydrocarbon reservoirs can be provided only if more and detailed results are reported. An extensive survey was carried out above a tight gas reservoir and an adjacent exploration area in Mexico. Data from several hundred stations with three-component broadband seismometers distributed over approximately 200 km 2 were used for the analysis. Several hydrocarbon reservoir-related microtremor attributes were calculated, and mapped attributes were compared with known gas intervals, with good agreement. Wells drilled after the survey confirm a predicted high hydrocarbon potential in the exploration area. A preliminary model was developed to explain the source mechanism of those microtremors. Poroelastic effects caused by wave-induced fluid flow and oscillations of different fluid phases are significant processes in the low-frequency range that can modify the omnipresent seismic background spectrum. These processes only occur in partially saturated rocks. We assume that hydrocarbon reservoirs are partially saturated, whereas the surrounding rocks are fully saturated. Our real data observations are consistent with this conceptual model.

Application of spectral decomposition to gas basins in Mexico

Recent reservoir studies involving spectral decomposition on various data sets from the Burgos and Macuspana basins of Mexico document the usefulness of this method as another way to uncover the effects of hydrocarbon accumulations on seismic data. Three such effects are illustrated in this article; attenuation of seismic waves passing through the reservoir, preferential reservoir illumination, and differential reservoir reflectivity.

A hydrocarbon microtremor survey over a gas field: Identification of seismic attributes

Erik H. Saenger*, ETH Zurich and Spectraseis, Arnaud Torres, Spectraseis, Susanne Rentsch, FU Berlin, Marc Lambert and Stefan M. Schmalholz, ETH Zurich, Efrain Mendez-Hernandez, Pemex Exploracion y Produccion Summary Narrow-band, low-frequency (from ~1H to ~6Hz) microtremor signals have been observed worldwide at the surface over hydrocarbon reservoirs (oil, gas and water multiphase fluid systems in porous media). These lowfrequency ëhydrocarbon microtremorsí possess remarkably similar spectral and signal structure characteristics, pointing to a common source mechanism, even though the depth (some hundreds to several thousands of meters), specific fluid content (oil, gas, gas condensate of different compositions and combinations) and reservoir rock type (such as sandstone, carbonates, etc.) for each of those sites are quite different. In this extended abstract we describe some results of an extensive low-frequency hydrocarbon microtremor survey carried out in Mexico. We describe how we extract a hydrocarbon potential map from spectral anomalies and compare this map with available reservoir location data. In a detailed analysis of two representative stations of the survey we extract different seismic attributes. Those attributes are compared with a theoretical model which can explain the source mechanism of hydrocarbon microtremors Introduction A growing number of surveys at different oil and gas field locations throughout the world have established the presence of ëhydrocarbon microtremorsí with a high degree of correlation to the location and geometry of hydrocarbon reservoirs (Dangel et al., 2003; Holzner et al., 2005; Graf et al., 2007 and references therein). These tremors can be used as a direct hydrocarbon indicator for the optimization of borehole placement during exploration, appraisal and production. The ever-present seismic background noise of the earth (e.g., Berger et al., 2004) acts as the driving force for the generation of hydrocarbon indicating signals. In contrast to conventional 2D and 3D seismic technologies, the investigation of ëhydrocarbon microtremorsí is entirely passive and does not require artificial seismic excitation sources. In this paper we describe some results of a specific survey carried out by Spectraseis over a gas field in Mexico. By using ultra-sensitive, portable 3C broadband seismometers (sensor type: G ̧ ralp) more than 500 measurements of the ever-present seismic wavefield at the surface were acquired over an area of approximately 200 km. We will briefly describe the main processing steps and the resulting hydrocarbon potential map. In a second step we analyse the recorded data at two locations in detail. We extract several key seismic attributes to characterize the hydrocarbon microtremor signal. The goal of this characterization is twofold: On the one hand this will solidify or falsify theoretical explanations of the origin of hydrocarbon microtremors. On the other hand the knowledge of those attributes will improve the processing and interpretation for future surveys. The survey: Acquisition, Processing and Interpretation The selected survey area lies in the Burgos Basin area in the north-western part of Mexico. The origin of this basin is associated to the opening of the Gulf of Mexico, during the Jurassic, starting the sedimentation in the Callovian with evaporitic deposits. The sedimentation conditions changed in the Cenozoic when a great regression occurred and a sedimentary sequence of at least 8000 meters thickness, with a great gas potential (represented by rocks with Type III Kerogen) was accumulated. The complex fault system in the surveyed area is made up of horst and graben structures, and is part of a large half-graben system (fault style is mainly listric). Minor faults compartmentalize the whole system in smaller blocks. A sensor grid layout is used with node spacing ranging from 250 to 1000 m. Several monitoring stations are installed for the duration of the entire survey. After each of the measurements (duration from 4 hours to 24 hours) the raw 3C sensor data (surface velocities) was stored with an individual identification number (id). The raw data may include strong perturbations (noises, artefacts) and discontinuities (data gaps). In order to obtain a clean signal in the time domain we cut out those time intervals with obvious strong artificial signals. From the cleaned data we calculate the power spectral density (PSD). An applied standard procedure is to determine the PSD for 40 sec. time intervals and to calculate the arithmetic average of each PSD for the whole measurement time. This leads to a stable and reproducible result in the frequency domain. In Figure 1 we show two spectra for the vertical component.

Advanced seismic techonology improves prospect evaluation and reservoir delineation in the mature Macuspana Basin, Mexico

An integrated study involving advanced seismic attribute and reservoir characterization tools in Macuspana Basin of Mexico provided an understanding of the structure, reservoir continuity, and quality of three mature reservoirs that optimized new drilling locations for exploration and development and generated a reservoir model for simulation purposes. Conventional and advanced technologies were integrated after a feasibility analysis to generate several attribute cubes over a 3D seismic volume; these included acoustic impedance, amplitude variation with offset (AVO), porosity derived from artificial neural network (ANN) schemes, and spectral decomposition. These attributes, added to the structural and stratigraphic analysis and to a J-function capillary pressure analysis, yielded information on compartmentalization that provided a geologic model to support reservoir delineation and characterization. Cobo, Bitzal, and Guiro fields are in Macuspana Basin, state of Tabasco, southeastern Mexico (Figure 1). The Cobo area, which includes the Cobo, Bitzal, and Guiro fields, is stratigraphically and structurally complex; its complexity is comparable to that of many other gas-producing fields in Macuspana Basin in which the dominant trap mechanisms vary from fault-assisted closures with a stratigraphic component to purely stratigraphic traps. The fields have produced gas for more than 20 years from a series of thin, principally Plio-Pleistocene sands thought to be fluvial-to-deltaic in origin. Cobo fields are cut by many NE-SW striking faults. Some northwest-dipping faults exhibit classic growth fault geometries; some show evidence for reactivation and inversion. This complexity, coupled with older well data, makes analysis using traditional exploration and reservoir characterization tools challenging. Figure 1. Map of Cobo area. Historically in Cobo fields, selection of well locations depended first on structural and stratigraphic analysis using limited 2D seismic; then, when 3D seismic became available, 3D seismic interpretation, detailed amplitude analysis, and AVO schemes were used as the basis for proposing new well sites. Amplitude, AVO, and resistivity measurements …