--> ABSTRACT: Improved Interpretations of Natural Fluid Inclusion Assemblages for Petroleum Exploration Applications Through Hydrothermal Experimentation, by Donald L. Hall, Richard E. Larese; #91020 (1995).

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Improved Interpretations of Natural Fluid Inclusion Assemblages for Petroleum Exploration Applications Through Hydrothermal Experimentation

Donald L. Hall, Richard E. Larese

Fluid inclusions provide a wealth of data about timing and conditions of petroleum migration, but there is presently a need for additional experimental data addressing controls on inclusion formation and preservation in the diagenetic environment. These will further our understanding of natural inclusion distributions, textures and strengths, and improve our ability to interpret natural assemblages with confidence. Furthermore, they will provide better fluid timing relationships and paleotemperature estimates, and allow a better understanding of hydrocarbon migration. Our method involves synthesizing fluid inclusions in rocks during laboratory-simulated burial and uplift with a hydrothermal apparatus and studying the inclusion assemblages obtained from these known, well-c nstrained conditions.

Hydrothermal cementation experiments using chlorite-coated, hematite-coated and uncoated sand grains, crushed and sieved Brazilian quartz and single quartz crystals produced fluid inclusions in quartz overgrowths and along dust rims. Inclusion formation was enhanced by the presence of impurities along the growth surfaces, and by rugose crystal surfaces. Results suggest that dust-rim inclusions may be inherently weak and susceptible to leakage and refilling with subsequent fluids. If gone unrecognized, data gathered on such altered inclusions could lead to misinterpretation of conditions accompanying cementation and relative timing of hydrocarbon emplacement. Cementation experiments at high oil saturation have shown that oil inclusions can form in the absence of obvious intergranular c mentation by exploiting pre-existing healed microfractures and filling old inclusion cavities. Hence active diagenesis and water-dominated pore fluids are not required for oil inclusion formation. Experiments at low hydrocarbon saturations suggest that even tiny amounts of oil can be trapped and recorded in the fluid inclusion record; no useful correlation was noted between oil/water phase ratios in entrapped fluids and the oil/water ratio of the experimental fluid.

Hydrothermal compaction experiments using crushed and sieved fluid-inclusion-free Brazilian quartz have shown that microfracture healing can occur in the absence of obvious intergranular cementation even at high oil saturation and can result in Inclusion distributions which are not visibly related to healed microfractures. The process appears to be accomplished by very local silica mobility which may be confined to the microfracture itself. These observations suggest that (1) fluid inclusion timing textures can be deceptive, and (2) active diagenesis is not requisite for oil inclusion formation along oil migration pathways.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995