A new pressure coring system has been developed which could significantly improve the measurement of in-situ oil and gas saturations in hydrocarbon reservoirs. This improved system incorporates a low-invasion fluid that is stored in the core retriever and is extruded to seal and protect the core. The low-invasion fluid consists of an extremely viscous, brine-based polymer system with sized calcium carbonate bridging particles to minimize invasion. This special coring fluid has been selected from several candidates that were laboratory tested at simulated downhole conditions. Both static and dynamic filtration characteristics were considered. Other important properties of this low-invasion pressure coring fluid are discussed, including freezing point, formation damage, clay stabilization, wettability changes, and pressure buildup in the core barrel. Preliminary results from actual coring operation are also presented.