2.3.1. Basin Pressure Compartments

On 30 July 1987 Dave Powley (Amoco Production Company) made a presentation to the Gas Research Institute (GRI) in Chicago entitled "Subsurface Fluid Compartments" [47]. His presentation described how, over the previous two decades, Amoco studies by John S. Bradley and himself showed that basins are commonly over-pressured (but sometimes under-pressured) and compartmented at depths greater than 3 and sometimes just 1 km. The pressure is di fferent between compartments, but the gradient is hydrostatic within compartments. He described the compartments as "huge [beer] bottles [which have been variably shaken]. Each one has a thin, essentially impermeable, outer seal and an internal volume which exhibits e ffective internal hydraulic communication." Figure 6A reproduces his presentation figure. He stated: " ... the compartments have an amazing longevity" and can " ... cut indiscriminately across structures, facies, formations, and geologic time horizons ... ". He gave examples from Romania, Norway, Burma, and Alaska. He urged GRI to investigate the causes of basin pressure compartmentation.

Over the next 10 years, the Gas Research Institute funded research on basin compartmentation and sealing. The first phase independently confirmed pressure compartmentation. AAPG Memoir 61 [48] documented that basins are commonly pressure-compartmented. Its poster child example is the Anadarko [49]. A highly over-pressured, gas-rich (20 tcf), ~240 × 113 × 5 km thick portion of this basin is overlain and underlain by normal hydrostatic pressures, as illustrated in Figure 6B. This zone has been overpressured for >250 Ma. It is so internally compartmented that Zuhair Al-Shaieb termed it a megacompartment complex. Compartments are nested within compartments in an almost fractal fashion.

**Figure 6.** (**A**) Buried bottle illustration of basin pressure compartmentation from Powley's 1987 address to the Gas Research Institute (GRI) [47]. (**B**) Overpressure in the Anadarko Basin (gray) from [49]. ( **C**) Top of overpressure (12 pound per gallon mudweight) from 2131 wells in the GRI Corridor. Deepest portion of the surface lies about 3.8 km under central Louisiana. ( **D**) topographic highs in the top of overpressure tend to underlie oil reservoirs. For relation in full Corridor see [12].

That overpressure compartments can cross cut stratigraphy and have irregular surfaces is illustrated by Figure 6C, which shows the top of overpressure in the o ffshore Louisiana GRI Corridor (located in the insert in Figure 6C). The top of overpressure (TOOP) is defined in this figure by the 12 lb/gal mud weight surface (lithostatic is ~22.7 lb/gal) interpolated from mud weights in the header logs of 2131 wells and Krieged to produce the surface shown. The TOOP transgresses from 112 Ma Cretaceous to 2.4 Ma Quaternary strata as it shallows toward the continental slope from ~3.8 km under central Louisiana to <1 km depth on the slope. The TOOP surface is highly irregular, with topographic highs rising ~1km from the baseline surface. The topographic highs are spatially associated with discovered oil fields (Figure 6D; [12]). Twenty of 29 hydrocarbon fields in the Corridor are near topographic highs in the TOOP.
