GENESIS AND DYNAMICS OF BASIN COMPARTMENTS AND SEALS

Observations and geochemical modeling studies confirm and extend the p roposition by Powley and Bradley that the deeper parts of a sedimentar y basin can be divided into hydraulically isolated compartments. It ap pears that there is a variety of mechanisms that yield this compartmen tation on a wide range of spatial scales. These effects imply the exis tence of a number of distinct types of compartments and nature of the sealing rock surrounding them. The main theme of the present article i s the range and interplay of the various scales of compartment and sea ling structures and the geochemical feedback that underlies much of th eir genesis and dynamics. On the largest (basin-wide) scale, the syste m may form a mega-compartment with transbasinal top seal and (strata a ssociated) basal seal. On the shortest scale (millimeters) the basin m ay become organized into arrays of stylolites or banded diagenetic com paction/cementation alternations. A host of compartmentation phenomena on scales intermediate to these extremes also exists. These diverse p henomena have been observed in the Anadarko basin and analyzed using f luid pressure-depth data, well logs, petrologic analyses of cores, opt ical and photo-illuminescent petrography, and isotopic and fluid inclu sion data. Reaction-transport-mechanical models show that some compart mentation may arise through self-organization via feedback phenomena. Hence the pattern of basin compartmentation need not simply be the res ult of an imposed ''template'' such as stratigraphy or faults. Rather, many of these phenomena arise out of the amplification of textural co ntrast via the aforementioned feedback. The feedback and other compart ment-forming processes are apparently embedded in the suite of well es tablished diagenetic processes. These include chemical reactions, mate rial and energy transport, and mechanical (notably pressure solution a nd fracturing) effects. It is the coupling between these processes tha t allows for many of the compartment phenomena of interest. From our m odels and observations we conclude that the very novel feature of comp artments is not only that there exist domains of hydraulically isolate d rock. Rather, it is that some compartments and the internal structur e of the seals that bound them may form spontaneously through geochemi cal self-organization embedded in the network df diagenetic reaction, transport, and mechanical processes. In this way, some compartments ha ve formed via an auto-isolation dynamic that could not have been predi cted on the basis of classical notions of seals related to strata ana faults. The new types of seals we identified arise via a number of mec hanisms as follows: (1) the instability of the uniform state of compac tion to the formation of banding via mechano-chemical feedback; (2) th e tendency for the periphery of a stratum or other body undergoing com paction (and thereby overpressuring) to become isolated from surroundi ng lower pressure regions; and (3) displaced precipitation or grain co mminution bands arising through the interplay df pressure solution and transport in systems with grins whose free faces are clay coated or o therwise kinetically inhibited. These seals, the nature and dynamics o f (migrating and stationary) seals, and episodic fluid migration and f ault-related sealing processes are discussed. The implication of our s tudies is that at depth in a sedimentary basin the rock-fluid system m ay display a strong tendency toward compartmentation and complex dynam ics, These findings have important Implications for our understanding of processes occurring deep in a sedimentary basin and for mineral and petroleum exploration and engineering.