Expulsion and Primary Migration of Hydrocarbons — Observations in Outcrops

Abstract

Hydrocarbons efficiently expel from fine-grained organic-rich source rocks upon thermal heating. Expulsion and primary migration through hydraulic fractures is a commonly cited process, but good outcrop examples are few. Occasionally, hydraulic fractures are permanently filled with, e.g., bitumen or fibrous carbonate cement, and we have studied them in the Green River Fm, Utah, USA, and the Vaca Muerta Fm, Neuquén Basin, Argentina. The aims of this paper are to show the visual expressions of expulsion and primary migration and to discuss associated mechanisms. Within the organic-rich Mahogany Zone (Green River Fm) are layer parallel fractures (max 1cm thick) filled with massive black bitumen. Immediately above the Mahogany Zone are vertical, near parallel, regularly spaced (0.5-1m) thin fractures (mm) filled with bitumen. Thickness increases and regularity decreases upward through the overlying fine-grained rock. In reaching the overlying Uinta Fm sandstone, the bitumen-filled fractures are up to 5m wide and extend for up to 20km. Vertical bitumen-filled fractures also initiate at the base of the Mahogany Zone and widen downwards to meter thick fractures. In the Vaca Muerta Fm are numerous layer-parallel fractures filled with fibrous calcite that contains hydrocarbon inclusions. These range from mm to cm in thickness and from cm to tens of meter in length. Vertical bitumen-filled fractures are observed above and below the organic-rich zone. One several meters wide fracture comprises Vaca Muerta source rock blocks in bitumen matrix. Short and wide layer-parallel hydro-fractures and organic-rich shale within fractures show that these layers were ductile. This is consistent with experiments where ductility increases with organic content and temperature. The transformation from general stress conditions (solid grains carry the weight of the overburden, σ1>σ2>σ3) to a plastic rock (isotropic stress, σ1=σ2=σ3) also shifts the fluid pressure to lithostatic. The combined effect of isotropic stress, anisotropic rocks, high fluid pressure and seepage force cause layer parallel fractures to open within organic-rich shales. The high intra-source rock fluid pressure acts on the surrounding shales where general stress conditions guides formation of vertical hydro-fractures. Hydrocarbons migrate through these fractures to permeable rocks. Increasing temperature causes the softening of the organic rich-shales and we therefore term this model “The thermal expulsion engine”.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016