The high-pressure and high-temperature (HPHT) areas of the central North Sea constitute an important hydrocarbon province. This includes the deep, Mesozoic reservoirs in United Kingdom quadrants 22, 23, 29, and 30. This study was undertaken to better understand oil and gas compositional histories in HPHT hydrocarbon systems and to help identify new exploration opportunities. The Late Jurassic Kimmeridge Clay Formation has been the source for both oil and gas over the entire area, with additional gas charge from the humic coals of the Middle Jurassic Pentland Formation in the western graben areas. The southern For-ties Montrose high, with its southward-plunging Mesozoic terraces, is host to numerous oil and gas fields with temperatures ranging from 90 to 180degreesC and formation pressures whose gradient to the surface exceeds 0.8 psi/ft (0.192 MPa/m). Several of these oil accumulations have undergone in-reservoir thermal cracking, resulting in a lighter, single-phase fluid, together with a pyrobitumen residue in the pore volumes. With several traps at or near their leak-off pressure, the likelihood of top seal failure and gas leakage is prevalent. Such top seal failure is intermittent and, in some instances, is associated with gas chimneys. The main causes of pressure increase in Mesozoic sediments are thought to be volume increases associated with gas generation from source rocks, clay dehydration, and thermal cracking of oil. Top seal failure because of pressure buildup by salt diapirism and the buoyancy of large hydrocarbon columns has resulted in a series of compositionally fractionated oils and gases. A new technique is presented, whereby the geochemical character of a shallow (Tertiary) oil reservoir that has undergone fractionation can help lower the risk of detecting the presence of hydrocarbon at depth in potential, deep (Mesozoic) reservoirs.