Time‐lapse crosswell seismic data acquired with a cemented receiver cable have been processed into P‐ and S‐wave tomograms which image heavy oil sand lithofacies and changes as a result of steam injection. Twenty‐seven crosswell surveys were acquired between two wells over a 3.5 month period before, during, and after a 34‐day, 30 MBBL [Formula: see text] steam injection cycle. Interpretation was based on correlations with reservoir data and models, observation well data, and engineering documentation of the production history and steam cycle. Baseline S‐ and P‐wave tomograms image reservoir sand flow units and areas affected by past cyclic steam injection. S‐wave tomograms define lithology and porosity contrasts between the excellent reservoir quality, “high flow” turbidite channel facies and the interbedded “low to moderate flow” bioturbated levee facies. The reservoir dip of approximately 20° is defined by the velocity contrast between lithofacies. P‐wave baseline tomograms image lithology, porosity, structure, and several low velocity zones caused by past steam injection. Previous steam‐heat injection caused the formation of gas which reduced velocities as much as several thousand ft/s (600 m/s), an amount which obscures the velocity contrast between lithofacies and smaller velocity reductions as a result of temperature alone. Time‐lapse and difference P‐wave tomograms document several areas with small decreases in velocity during steam injection and larger decreases after cyclic steam injection. Velocity reductions range from 300 to 900 ft/s (90 to 270 m/s) adjacent to and above injectors located 20 to 50 feet (6 to 15 m) from the tomogram cross‐section. Poisson’s ratio tomograms show a significant decrease (.10) in the same area, and include low values indicative of gas saturation. Continuous injectors located 50 to 350 feet (15 to 100 m) from the survey area also caused a progressive decrease in velocity of the “high flow” channel sands during the time‐lapse survey. Interdisciplinary interpretation indicates that tomograms not only complement other borehole‐derived reservoir characterization and temperature monitoring data but can be used to quantitatively characterize interwell reservoir properties and monitor changes as a result of the thermal recovery process. Monitoring results over 3.5 months confirms that stratification has controlled the flow of steam, in contrast to gravity override. This suggests that tomographic images of reservoir flow‐units and gas‐bearing high temperature zones should be useful for positioning wells and optimizing injection intervals, steam volumes, and producing well completions.
The Cagayan basin of Northern Luzon, an interarc basin 250 km long and 80 km wide, contains a 900 m thick sequence of Plio‐Pleistocene fluvial and pyroclastic deposits. These deposits are divided into two formations, the Ilagan and Awidon Mesa, and three lithofacies associations. The facies, which are interpreted as meandering stream, braided stream, lahar, and pyroclastic flow and fall deposits, occur in a coarsening upward sequence. Meandering stream deposits interbedded with tuffs are overlain by braided stream deposits interbedded with coarser pyroclastic deposits; lahars and ignimbrites. The coarsening upward volcaniclastic deposits reflect the tectonic and volcanic evolution of the adjacent Cordillera Central volcanic arc. Uplift of the arc resulted in the progradation of coarser clastics further into the basin, the development of an alluvial fan, and migration of the basin depocentre away from the arc. The coarsening of the pyroclastic deposits reflects the development of a more proximal calc‐alkaline volcanic belt in the maturing volcanic arc. The Cagayan basin sediments serve as an example of the type and sequence of non marine volcaniclastic sediments that may form in other interarc basins. This is because the tectonic and volcanic processes which controlled sedimentation in the Cagayan basin also affect other arc systems and will therefore control or significantly influence volcaniclastic sedimentation in other interarc basins.
Index map of the central Cagayan Valley Tectonic map of the Luzon-Taiwan region Island arc evolution of Northern Luzon Summary of events in the island arc evolution of Northern Luzon from the Eocene to the present Schematic island arc evolution of the Cagayan basin Migration of the Cagayan basin axis in Miocene through Pleistocene Geologic map of the central Cagayan Valley Graphic sections of the llagan Formation Photographs of the llagan and Awidon Mesa formations Graphic sections of the Awidon Mesa Formation, Tabuk plateau, and Pangul Anticline Graphic sections of the Awidon Mesa Formation, Cabalwan and Enrile anticlines Tektite locality map Textural classification of the conglomerate matrix samples Probability plots of braided stream and debris flow deposits and plots of Awidon Mesa Formation conglom erates interpreted as braided stream and debris flow deposits Morphology of central Cagayan Valley tektites Compositional classification of conglomerate matrix samples Probability plots of density and debris flow deposits and Awidon Mesa Formation tuff-breccia samples interpreted as density and debris flow deposits Page 70 108 188 ix LIST OF TABLES Stratigraphie nomenclature of the Cagayan Valley Grain size statistics of selected conglomerates, tuff-breccias, tuffs, and sandstones Clast llthologles of selected conglomerates and river gravels Major and minor element composition of a Cagayan Valley tektite and average Philippine tektite composition Conglomerate matrix composition and mineralogical components of the framework grains Composition of tuff-breccia matrix and tuffs Modal refractive indices of Awldon Mesa Formation tuff and tuff-breccia pumice fragments or glass shards Sandstone composition and mineralogy Qualitative mineralogy of sandstone clay minerals determined by X-ray diffraction Heavy mineral data in number percent for Cagayan basin sandstones and river sands Heavy mineral associations of the Upper Member of the Ilagan Formation, Awldon Mesa Formation, and Holocene river sands Statistical parameters of selected mudrocks Qualitative mineralogy of mudrock samples Lithofacles codes and llthofacles of the Ilagan and Awldon Mesa Formations Characteristics of proximal and distal Ignlmbrltes Summary of Pleistocene climatic interpretations for Southeast
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