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Petroleum Geochemistry

Competency Statement:

Upon completion of this module the learner will understand geochemical concepts relating to the origin and occurrence of petroleum. Describe oil and gas chemical composition and classification. Identify source rock based on geochemical analysis. Interpret petroleum migration from source rock to subsurface hydrocarbon accumulation.

Learning Objective:

· Understand chemical composition and classification of oil and gas.

· Describe the types of kerogen and the generation, expulsion and migration of petroleum.

· Use principal tools and methods for evaluation of source rock quality and maturity.

· Evaluate one-dimensional geochemical model.


Assignment Instruction:

As a member of the New Ventures Exploration Team, you are assigned to evaluate a regional integrated geochemical study of a sedimentary basin relevant to hydrocarbon habitat. The study includes source rock evaluation, oil geochemistry and their correlation with source rock, reconstruction of the hydrocarbon generating area and geochemical modeling of a pilot well.

The drainage area of the basin is bounded to the north and northeast by faults, and to the southeast, south, and west by mountains (Figure 1). The tectonic setting of the basin evolved during the Cretaceous Period and the Cenozoic Era. The present structural configuration of the basin developed during the Miocene to Holocene Epochs as it became an intermontane basin in a foredeep position.

An Aptian to Albian Age marine transgression deposited a thick shallow water carbonate platform and associated sediments. During maximum transgression between the Cenomanian Age and the Coniacian Age, the sedimentation was pelagic and euxinic facies of limestones and calcareous shales. During Paleocene Epoch, clastic sediments were deposited in the southwest and western basin areas and a shallow marine platform occurred in the central basin. During the lower to middle Eocene Epoch, the sedimentation was mainly fluvial in the southwest, fluvio-deltaic to deltaic on the platform, and turbidite and flysch in the east.

Geochemical analyses of cores and cuttings from nearly 100 wells included TOC, pyrolysis, visual kerogen, UV, GC, GC-MS biomarkers, and TTI. The important oil prone source rocks were deposited during the Cretaceous. Organic carbon (OC) ranges between 1.5% and 9.6% (average of 3.8%) in limestone and calcareous shales with high concentrations of extractable organic matter (>2000 ppm) and hydrocarbons (>1000 ppm). The Paleocene and Eocene sequences in the basin contain mainly gas prone type III organic matter with low potential.

 Figure 1 - Location of wells studied in the basin

Location of the oil wells