Several mayor gas discoveries have been done in the Guajira Basin, among them: the Chuchupa (~ 3500 GCFG),Ballena (~ 1257 GCFG), Riohacha (~ 92 GCFG) and the Santa Ana area (400 GCFG-15 MCFGD) fields. The regional evaluation conducted by Ecopetrol (1998), indicates a yet-to-find potential of 2800 MMBOE, make up of 70% gas and 30% oil.
I. Upper Guajira Sub-basin
A great oil generation capacity is expected in the Guajira basin due to the presence of La Luna Formation, composed of shales, calcareous siltstones and limestones (Figure 2). The La Luna Formation has been identified in cores of PGG 1 well and east of the Cocinetas Depression in Venezuela. The generation potential has been also confirmed from Paleogene and Neogene samples, with dominant kerogen type III, i.e. gas prone.
Important reservoirs are supposed to be found within limestones and sandstones of Siamaná and Macarao formations present in Cocinetas-1 and Guajira-1 wells and reaching thicknesses between 200 and 300 m.
Good quality seals are expected in calcareous mudstones and mudstones occurring near the base of Siamaná Formation; its thickness varies between 300 and 1000 feet.
In the onshore part of the basin, most of the structures were formed during late Paleogene- early Neogene times. Secondary migration occurred soon after the Late Neogene first phase of restructuration.
The structural configuration of the offshore portion might have improved the hydrocarbon’s generation and migration favoring the migration ways from an early thermogenic source in the deep offshore towards Chuchupa, Ballena and Riohacha fields.
Trapping mechanism consists of monocline structures dipping south, the closure being made of extensional faults with south vergence and the west and east closure formed by the wedging of the Macarao Formation against a basement high (Figure 3).
II. Lower Guajira Sub-basin
Hydrocarbons have been found in different intervals of the Paleogene and the Neogene units (Chuchupa-Ballena field). The existence of hydrocarbons derived from La Luna Formation has been proposed, as another source existing mainly in the eastern part of the basin.
Sandstones and limestones common in the Jimol and Uitpa formations and deposited in shelf environments constitute good reservoirs with thicknesses between 400 and 1800 feet.
Mudstones and shales from Jimol and Uitpa formations are the main seal rock.
There are two types of traps, stratigraphic and structural; the stratigraphic ones are determined by onlap of sedimentary sequences over the basement; the structural traps are generated by reactivation of pre-Neogene faults system located at the south of the sub-basin (Figure 3).