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

José Alejandro Méndez Dot,José Méndez Baamonde,Rommel Whilchy,Dayana Reyes

doi:10.1590/2317-4889201530192

José Alejandro Méndez Dot, José Méndez Baamonde + Show 2 more

Open Access

https://doi.org/10.1590/2317-4889201530192

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Abstract

ABSTRACTCarbonates 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.

Highlights

The Cogollo Group is a shallow carbonate platform composed by many sedimentological facies and depositional textures
Sampling covered the stratigraphic intervals of interest in six cores of six wells located at the Maracaibo Basin, and this was done based on the following criteria: ■■ Visual estimates as the colour of the rock, lamination degree, grain size and absence of bioturbation. ■■ Correlation between the depth of the rock intervals of interest in both electric logs and cores (Fig. 8)
All samples were administered through the Rock-Eval 6 pyrolysis technique, in order to obtain the values of: total organic carbon (TOC) (% weight), S1, S2, S3, and T Max

Summary

INTRODUCTION

The Cogollo Group is a shallow carbonate platform composed by many sedimentological facies and depositional textures. Aptian (~ 120 Ma), represented by the Machiques Member, and the OAE 1b of the late Aptian (~ 113 Ma), represented by the Piché Member, both of them belong to the Apón Formation (Méndez et al 2008) These two bituminous intervals, whose correlative sections have been studied in many sedimentary basins in Europe, the Mediterranean sea, Florida (USA), Gulf of Mexico, the Pacific and the Atlantic oceans (Sliter 1989; Bralower et al 1993, 1994; Jenkyns & Wilson 1999; Leckie et al 2002; Dumitrescu et al 2006; Hofmann et al 2008; Robinson et al 2008), represent two effective source rock levels.

COCINAS CERRAJON SAN FRANCISCO DELTANO

ORGANIC GEOCHEMISTRY RESULTS

CONCLUSIONS