Larese, R.E. and Hall, D.L.
Amoco Tulsa Technology Center, P.O. Box 3385, Tulsa, OK 74102-3385
Sieved, angular Brazilian quartz grains were compacted at 360oor 150oC for 288 hr. with an effective stress (confining pressure - pore pressure) of 690 bars in the presence of a pore fluid consisting of 100% natural crude oil. The post-compactional sample displayed negligible pressure solution or quartz cementation but prominent physical compaction effects, notably an abundance of open microfractures and crushed quartz grains. In spite of the lack of intergranular cementation, and low total water content of the system, many microfractures generated during the experiment healed in the presence of the oil pore fluid, and these are decorated with oil inclusions. The water content of the oil was determined prior to experimentation to be about 0.1 wt%; hence, even adding the adsorbed, atmospheric-derived water which inevitably coats the interior of the capsule and quartz grains, the total quantity of water within the system was small--far below that expected in virtually all natural systems. In addition to oil inclusions along healed microfractures, other oil inclusions appear to have exploited pre-existing aqueous inclusions, apparently through breaching, flushing and refilling of old inclusion cavities. This "exploitation" process appears to be energetically favored at lower temperatures as compared to fracture healing.
Cementation experiments were conducted at 325oC and 690 bars of hydrostatic pressure using a combination of angular Brazilian quartz, rounded quartz sands and chlorite or hematite coated quartz sands, along with a variety of pore fluids ranging from 100% aqueous to 100% oil. Texturally primary fluid inclusions were rare within overgrowths and along dust rims, and no preference was noted for inclusion formation along coated or uncoated grains. Exploitive oil inclusions, however, were common within detrital portions of quartz grains. Some of the rare dust-rim inclusions were incompletely sealed during the experiments, resulting in porous, permeable interfaces which were accessible to later pore fluids in spite of significant overgrowth formation. We were able to introduce oil into these dust-rim pores, thereby producing a false texture of primary oil inclusions along quartz overgrowth boundaries.
Brazilian quartz and Phosphoria shale (containing 20% total organic carbon) were mixed in a 9:1 ratio (giving a 2% total organic carbon bulk sample) and compacted in an aqueous pore fluid at 360oC with an effective stress of 690 bars for 336 hr. Generation of oil and gas in-situ, and entrapment of this petroleum along healed microfractures resulted in oil and gas inclusions in quartz, most of which were not spatially related to the shale grains. The results of this and other experiments described above imply the following conclusions for interpreting petroleum inclusion textures and distributions in natural rocks:
1) Oil inclusions can form in the burial environment from oil-dominated pore fluids even in the absence of intergranular cementation and intense compaction, either by diffusive crack healing, or exploitation of old inclusion cavities. The documentation of petroleum inclusion formation mechanisms which do not rely on active cementation suggests that paleo-migration events are easier to document in the fluid inclusion record than previously believed.
2) Textural criteria can be misleading. Exploitive inclusions with no visible fluid entry points may be mistakenly identified as inherited or pre-overgrowth inclusions. Infiltration and entrapment of oil along weak boundaries between detrital grains and overgrowths can cause deceptive timing relationships and inaccurate interpretations of reservoir quality at the time of migration.
3) Oil and gas inclusions can be formed through in-situ generation/entrapment processes in nominally non-source intervals if high TOC shale fragments are present. The resulting textures may not be distinguishable from migrated hydrocarbons; hence, thermal maturity must be taken into consideration when interpreting petroleum inclusion distributions.