Case History Application of seismic attributes and spectral decomposition for reservoir characterization of a complex fluvial system: Case study of the Carbonera Formation, Llanos foreland basin, Colombia Lucia Torrado 1 , Paul Mann 1 , and Janok Bhattacharya 2 ABSTRACT Hydrocarbon exploration in the Llanos foreland basin of eastern Colombia has traditionally focused on structural traps. However, in the past decade, the country’s oil demand has gen- erated an increased interest in exploration for stratigraphic traps. We integrated 700 km 2 of 3D seismic data volumes with eight wells in the Central Llanos area to assess the reservoir potential of the fluvial channel deposits of the Late Eocene–Oligocene Carbonera Formation in the Casanare Province. Distinguishing nonproductive, mud-filled channels from productive sand-filled channels is of economic impor- tance for hydrocarbon exploration because both channel types can exhibit a similar seismic character. Our interpretation of the fluvial sandstone and the reservoir identification was based on 3D seismic attributes, including coherence, curvature, and spectral decomposition, and the analysis of fluvial geomor- phology. Analysis of stratal slices through coherence, isofre- quency amplitude cubes, and curvature cubes revealed (1) a northeast-trending meandering fluvial system with changes in the rivers’ paleoflow direction from southwest to northeast during the period of the Late Eocene to the Early Miocene, (2) development of prospective sandy point bars, scrolls, and sand bar deposits, and (3) channel ridges and bases accen- tuated by differential compaction dependent on the channel’s filling material. Based on our attribute analysis, we were able to characterize nonprospective mud-filled channels versus pro- spective sand-filled channels. INTRODUCTION The Llanos foreland basin of northeastern Colombia is the coun- try’s premier oil-producing basin with a year 2011 production of 625,000 barrels of oil per day found mostly in normally faulted structural traps with reservoirs consisting of fluvial sands from the Eocene epoch to the Oligocene epoch (Barrero et al., 2007). Because the belt of Eocene-Oligocene fluvial channels extends over an area of 75;000 km 2 in the Llanos foreland basin and is contained in the Carbonera Formation that averages 762 m in thickness (Ecopetrol, 1995), the probability of discovery of future strati- graphic traps within sand-filled, fluvial sands is high — provided the explorationist can distinguish sand-filled channels from mud- filled channels in the subsurface. In recent years, a wide variety of geophysical techniques have been developed to image plan views of the depositional environment and other geologic features extracted from 3D seismic volumes. These methods have developed into a rapidly evolving discipline known as seismic geomorphology that has also become an important tool for oil and gas exploration (Davies et al., 2007). Stratal slices and horizon slices from 3D volumes can be studied using different seis- mic attributes that highlight seismic facies variations within the 3D seismic cube, which in turn may be interpreted as depositional sys- tems, geomorphologic features, faults, and fracture patterns, and can ultimately be used to predict lithologies (Davies et al., 2007). The well-log and seismic response of these stratigraphic features provide important tools to identify reservoir facies and hydrocarbon trapping configurations and thus reduce drilling risk in frontier areas. Manuscript received by the Editor 15 November 2013; revised manuscript received 15 May 2014; published online 22 August 2014. 1 University of Houston, Department of Earth and Atmospheric Sciences, Houston, Texas. E-mail: lucia.torrado@gmail.com; pmann@uh.edu. 2 McMaster University, School of Geography and Earth Sciences, Hamilton, Ontario, Canada. E-mail: bhattaj@mcmaster.ca. © 2014 Society of Exploration Geophysicists. All rights reserved. B221 GEOPHYSICS, VOL. 79, NO. 5 (SEPTEMBER-OCTOBER 2014); P. B221–B230, 10 FIGS., 1 TABLE. 10.1190/GEO2013-0429.1