BEJARANO, CELIA, Corpoven, S.A., Puerto La Cruz, Venezuela

ABSTRACT: The Unare Lagoon: A Recent Example of Sequence Stratigraphic Control in Reservoir Patterns

The Unare lagoon constitutes a barred coastal lagoon formed during the Holocene transgression on a moderate wave energy, microtidal coast. An extensive surface sampling and core drill program has been carried out in the lagoon in order to-develop a reservoir sedimentology and sequence stratigraphic model applicable to similar subsurface deposits. During the rapid Holocene sea level rise, more than 70 m of fluvial and delta plain sediments have aggraded behind the landward stepping coastal barrier. These sediments are truncated seaward by a transgressive wave ravinement surface, and are capped by widespread lagoonal muds which accumulated between 8300 and 7250 yBP. These muds constitute the Holocene Maximum Flooding Surface which preceded the onset of the post Holocene stillstand (c. 5 0 yBP), probably as a result of the high rate of sediment supply and the confined nature of the lagoon which acted as an efficient sediment trap. During the post Holocene stillstand, a fluvial-dominated delta has prograded across the lagoon and attained the coastal barrier. This delta constitutes a Highstand Systems Tract. The major reservoir sands comprise distributary channel meanderbelts and the transgressive barrier. The channels form sand ribbons 5 to 7 m thick, and up to 2 km wide. The barrier and shoreface sands forms a strike-elongate deposits less than 5 m thick, up to 150 to 600 m wide, and 5 to 10 km long. The lagoonal facies of the Maximum Flooding Surface form a good reservoir seal overlying the aggrading transgressive fluvial and delta plain sands and muds.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.