Integrated Multiscale Approach for Lithological and Mineralogical Characterization of Unconventional Shale Plays: Understanding the Early Cretaceous rocks in La Luna-1 well, Middle Magdalena Valley Basin, Colombia

Galvis-Portilla, Henry A.¹; Marfisi, Nelbett; Higuera-Díaz, I. Camilo; Ballesteros, Cesar Ivan; Cespedes, Sandra Patricia; and Marin, Maria del Pilar
¹[email protected]

The fine-grained Cretaceous sequence in the Middle Magdalena Valley Basin of Colombia are among the richness in organic content and best hydrocarbon-generating rocks in the world. However, for all this good generating characteristics very few studies have addressed the potential of these rocks as an unconventional hydrocarbons reservoir. To characterize the potential as a reservoir of these rocks we developed a comprehensive methodology to understand the lithological and mineralogical similitudes and heterogeneities from cm- to nm- scale that may define and affect the storage ability and frackability of fine-grained rocks in this region. We use a combination of core description, thin section petrography, XRD and SEM analyses to describe the recovered core from La Luna-1 well.

From the 3800 ft. of core recovered, we characterized systematically from cm- to nm-scale lithological and mineralogical features to understand facies arrangements at different scales. Here we focus on a 500 ft. interval from the Barremian to lower Albian that typifies variability on facies arrangement. At cm-scale a lithological core description shows interlayering of dark-grey to black color laminated marlstones and limestones. Thin section description reveals a variety of components including laminated clay-size minerals, organic matter layers, silt-size quartz grains, bioclasts, and calcite, siliceous, and dolomitic cements. XRD results confirmed the bulk rock composition showing silica fractions up to 50 percent in weight in these carbonate mudrocks. SEM analyses helps determining that although there is a silt-size siliclastic fraction most of the silica appears on crystalline aggregates as cement with local and widespread silicification.

This variability in the lithological and mineralogical composition is common within a variety of organic-rich hybrid mudrocks that encompassed six lithofacies at cm-scale: siliceous mudstone, calcareous mudstone, silicified calcareous mudstone, dolomitic limestone, crystalline limestone, clay shale. These six lithofacies have in common that the composition is similar at mm- and nm-scale. However, the volumes and arrangement of each mineral and lithology are different among the six cm-scale lithofacies. Therefore, we suggest that for monotonous successions of fine-grained rocks it is more important to determine the ratios between basic components than to base predictions only in mineralogy and lithology descriptions.

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013