The Subandean basin is an arcuate structural depression aligned in a north-south direction between the Guyana shield and the Andes. Nevadan orogenic movements there were followed by miogeosynclinal conditions that characterized sedimentation throughout the Cretaceous Period. The initial deposits of an Aptian-Albian

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marine transgression were epicontinental sandstones, known as the Hollin Formation, which are the primary drilling objectives in the Subandean basin. As subsidence continued, the relief of the Guyana shield was reduced by erosion, and finer-grained uppermost Albian to Cenomanian sediments were deposited. These beds, called the Napo Formation, are mainly interbedded shale, glauconitic sandstone and bituminous limestone. They form an important oil-producing section. The source of Napo and Hollin sediments was on the east in the Guyana shield. The sub-Hercynian orogeny terminated the cycle of sedimentation and led to the beginning of deposition in a fluviatile and lacustrine environment. The predominantly freshwater environment persisted throughout Tertiary time and sediments were derive from the newly uplifted Andes on the west.

Commercial production was established in Colombia in 1967 and the producing trends have been extended south to the northern border of Peru. Oil has accumulated in mountain-front and mid-basin traps that are most commonly fault-associated anticlines.

All crude discovered has been of the "sweet" type. There is an increase in oil gravity from the eastern shelf to the western basin axis. Gas-oil ratios are low. No gas fields have been discovered.

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