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Seismic AVA as an Exploration Tool in the Magdalena Basin Offshore Colombia

Fasnacht, Timothy1 and Frost, Brian R.
1[email protected]

It is always exciting to analyze high-quality seismic data within an unexplored basin because of the potential discovery of vast resources. The provenance of the reservoirs within the deepwater Magdalena Basin is siliciclastics and igneous rocks from over 1000 km away. The Magdalena River and its associated tributaries drain a sediment resource area of >400,000 km². The depositional environment for these sediments is expected to be deepwater channel and fan complexes with present water depths ranging from about 550 – over 4000 m.

Seismic AVO or AVA (amplitude vs. offset or angle) has historically been especially effective when targeting relatively young, high porosity sandstones. Since the exploration targets in this area are expected to be Miocene sands, AVA is a primary exploration tool, albeit uncalibrated. AVA analysis for Block 2 (~12,000 km²) was based on a very sparse grid of recent regional 2D lines: three dip and three strike – totaling about 570 linear km. They were processed to preserve relative amplitude and controlled phase and are very close to zero phase – all essential elements for proper AVA analysis. Near (5-20), Mid (25-35) and Far (35-50) angle stacks were reviewed, plus two AVA attribute stacks derived from these angle stacks: (Far-Near)*Far and Absolute Value Far – Absolute Value Near. Direct analysis of the angle gathers was critical for a better understanding the quality of the input for the AVA attribute stacks. Examination of gathers is an essential component of any AVA-related prospect.

Potential exploration targets include both stratigraphic and structural traps, most with associated anomalous AVA signatures. The majority of these AVA anomalies are Class 3 and 2 (both having high amplitude on the Far angle stacks). Many have indications of stacked and potentially HC-bearing sands. The highest amplitude Class 3 anomalies may be more indicative of gas-prone, porous reservoirs, however a similar seismic response could also be produced by high GOR oil trapped within the pore space. There are also deeper Class 2P and Class 1 AVA signatures.

While our preferred interpretation of the anomalies is stacked pay, other scenarios can also generate Class 3 and 2 AVA signatures. Among the most common AVA pitfalls are: 1) high porosity, thick wet sands, 2) marls, ash or coal beds, 3) wet reservoirs with low saturation gas (Sw = 5-10%), 4) abnormally high impedance shales above the reservoir and 5) amplitude tuning thickness.

 

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013