drainage units with the flooding surfaces, which are overlain by shales or carbonate-cemented horizons, forming a barrier to vertical reservoir communications. Sequence stratigraphic principles have optimized production potential once reservoir scale architecture is identified and separate drainage units identified. Systems tracts The concept of systems tracts evolved to link the contemporaneous depositional systems. Systems tract forms subdivision in a sequence. Different kind of systems tracts are assigned on the basis of stratal stacking pattern, position in a sequence and in the sea level curve and types of bounding surfaces. A lowstand systems tract (LST) forms when the rate of sedimentation outpaces the rate of sea level rise during the early stage of the sea level curve. It is bounded by a subaerial unconformity or its correlative conformity at the base and maximum regressive surface at the top. A transgressive systems tract (TST) is bounded by maximum regressive surface at the base and maximum flooding surface at the top. This systems tracts forms when the rate of sedimentation is outpaced by the rate of sea level rise in the sea level curves. A highstand systems tract (HST) occurs during the late stage of base level rise when the rate of sea level rise drops below the sedimentation rate. In this period of sea level highstand is formed. It is bounded by maximum flooding surface at the base and composite surface at the top. Regressive systems tract forms in the marine part of the basin during the base level fall. Subaerial unconformities form in the landward side of the basin at the same time. Parasequences and stacking patterns A parasequence is a relatively conformable, genetically related succession of beds and bedsets bounded by marine flooding surfaces and their correlative surfaces. The flooding surfaces bounding parasequences are not of the same scale as the regional transgressive surface that is associated with a sequence boundary. The parasequences are separated into stacking patterns: Aggradational Progradational Retrogradational Each stacking pattern will give different information on the behaviour of accommodation space, a major control of which is relative level. So a rapidly progradational pattern will be indicative of falling sea level, rapidly retrogradational is evidence for rapidly transgressing sea level and aggradational will be indicative of gently rising sea level. Sea level through geologic time Sea level changes over geologic time. The graph on the right illustrates two recent interpretations of sea level changes during the Phanerozoic.