THE LATE CRETACEOUS M1 MEMBER OF THE NAPO FORMATION, A TIDAL DELTA SYSTEM AND THE IMPLICATIONS FOR THE GEODYNAMIC EVOLUTION OF THE ORIENTE BASIN OF ECUADOR

The Oriente Basin of Ecuador is one of the most prolific oil basins of northwestern South America. The oil is hosted in Cretaceous reservoirs deposited during the Aptian to Campanian interval. Oil exploration and production is mostly focused in the Albian - Cenomanian fluvio-estuarine sandstones of the Hollin and Napo formations (T and U cycles), deposited in response to sea level fluctuations in an epicratonic basin. The Campanian M1 Member of the Napo Formation is a poorly understood reservoir restricted to the eastern part of the Oriente Basin. This study presents a geological model for the M1 Member in the Eden Yuturi oil field, based on sedimentological, wireline logs and seismic data.

Sedimentological analysis of representative cored sections of the M1 member present three main lithofacies associations, from base to top: 1) Sharp-based turbidites intercalated with high-viscosity debris flows, interpreted as prodelta deposits; 2) Oil saturated quartz-rich sandstones with through cross stratification, grading, soft sediment deformation and asymmetric ripples caped by mud drapes, interpreted as channels with tidal influence in a delta front position; 3) Inclined heterolithic sandstones, with tidal rythmites, interpreted as point bars produced by lateral accretion of meandric channels of facies 2. This succession is interpreted as a shallowing upward deltaic system with a tidal influence.

Wireline logs correlations and RMS amplitud images show a wedge-shaped prograding system, whereas thickness maps of the M1 Member present lobate shapes that thin rapidly to the southeast of the field, pointing to a delta with a source region located to the northeast.

The time of delta progradation is coeval with a regional tectonic event related to the collision of the Caribbean Plateau to the west of northern South America. This event may have led to the uplift of the source regions of the M1 Member, which resulted in the westward progradation of delta lobes.

The potential for stratigraphic traps within deltaic system are high, due to the compartmentalization of the individual sandstone clinoforms. The importance of defining a tide dominated system for the M1 Member is that these systems tend to be large and can extend hundreds of kilometers across and along of continental margins, enhancing the oil prospectivity of this reservoir.