The Cogollo Group and the oceanic anoxic events 1a and 1b, Maracaibo basin, Venezuela

Abstract

ABSTRACT:Carbonates of Cogollo Group (Apón, Lisure and Maraca formations) constitute the broader calcareous platform system originated during Aptian and Albian of Cretaceous in north-western South America, Maracaibo Basin, Venezuela. On the shallow shelf, a variety of calcareous sedimentary facies were deposited during marine transgressive and regressive cycles. Some of them developed porosity and constitute important hydrocarbon reservoirs. Due to some major marine transgressions, from early Aptian, the anoxic environment and characteristic facies of a pelagic environment moved from the outer slope and basin to the shallow shelf, during specific time intervals, favouring the sedimentation of organic matter-rich facies, which correspond to the oceanic anoxic events (OAEs) 1a and 1b. The source rock of Machiques Member (Apón Formation) was deposited during early Aptian OAE 1a (~ 120 Ma). The source rock of Piché Member, located at the top of the Apón Formation, was deposited during late Aptian OAE 1b (~ 113 Ma). Finally, La Luna Formation, from Cenomanian, that covers the OAE 2 (~ 93 Ma), represents the most important source rock in the Maracaibo Basin. In this way and based on sedimentological and organic geochemistry results from the determinations performed on 247 samples belonging to six cores in the Maracaibo Basin, we propose these two organic-rich levels, deposited on the shallow shelf of the Cogollo Group, as "effective source rocks", additional to La Luna Formation, with oil migration in relatively small distances to the porosity facies. KEYWORDS:

Lamniform sharks from the Cenomanian (Upper Cretaceous) of Venezuela

“…The specimens described here were sampled from two horizons within the La Luna Formation: one from the Aguada Member (Cementos Andinos Quarry, Trujillo state) and the other from just above the contact between the Maraca Formation (Cogollo Group) and the La Luna Formation (La Luna Quarry, Zulia state). While a Cenomanian age was proposed for the Aguada Member based on foraminifera and ammonites (Renz, 1959), the presence of horizons of Cenomanian age in the Maracaibo Basin, from which the La Luna Quarry samples originate, has been controversial (Dot et al, 2015;Erlich et al, 1999;Renz, 1982). The two sampled localities have distinct (and rich) anacoracid composition but they share two taxa: M. saskatchewanensis and A. karsteni.…”