3-D Seismic Interpretation: Time-lapse seismic
We saw in chapter 5 how seismic data can be used in favourable cases to infer the nature of fluid fill (gas, oil or brine) in a reservoir. An application of this is to follow the way that fluids move through a reservoir during production, by carrying out a baseline seismic survey before production begins and then repeat surveys over the production lifetime. Where 3-D surveys are repeated in this way, they are often referred to as 4-D seismic , with the idea that time is the extra dimension over standard 3-D. Differences in seismic amplitudes or travel-times between the surveys can reveal the movement of fluid contacts (e.g. where produced oil has been replaced by brine) or the extent of pressure changes that affect reservoir properties. As sketched in figs. 8.1 and 8.2, it is not always straightforward to separate out the causes of the changes. At a producing well, the pressure drops, and if it drops far enough then gas will come out of solution. This will result in a decrease in both P velocity and in density, resulting in a drop in acoustic impedance. On the other hand, the drop in pore pressure causes an increase in P velocity and density. At a water injector (fig. 8.2), we replace oil by water, which in itself would increase P velocity and density; however, the injection also increases the pore pressure, which causes the reverse effect on P velocity and density.
Repeatability issues of 3-D VSP data
Increased repeatability is recognized as one major issue for improving the time-lapse seismic technology as a reservoir management tool. A 3-D vertical seismic profiling (VSP) data set, acquired over a period of two days, is used to analyze how repeatable a permanent installed geophone array can be and how repeatability changes with inaccuracies in source positioning. It is found that for a frequency range between 3.5 and 50 Hz, the difference root-mean-square (rms) level between two recorded traces belonging to two different shots is about 8%. This fact shows that there is a potential for acquiring very accurate time-lapse seismic data by using a permanently installed downhole geophone array. Repeatability variation with increasing shot separation distances is analyzed, showing a rapid decrease in repeatability as the accuracy of the positioning of the repeat survey decreases. Horizontal geophone components show approximately the same degree of repeatability compared to the vertical component, but horizontal geophone data is slightly more sensitive to positioning errors. The results show that repeated 3-D VSP surveys (preferably using permanently installed geophone arrays) might be an efficient tool for detailed and precise monitoring of fluid and pressure changes within a hydrocarbon reservoir.
Mesozoic rifting and basin inversion along the northern African Tethyan margin: an overview
Abstract The northern African Tethyan margin registered three major rifting episodes from the latest Palaeozoic-earliest Mesozoic to the earliest Cenozoic. Break-up of Gondwana was initiated in the late Carboniferous. Along the northern African-Arabian plate margin rifting propagated westward from the northeastern Arabian margin to Morocco during the Permian and Triassic, and was accompanied by Mid-Late Triassic-earliest Liassic extensive alkaline flow basalts. Rifting continued during the Liassic, e.g. in the Moghrebian Atlas troughs. A second stage of rifting occurred in the Late Jurassic and continued into, or was rejuvenated during, the Early Cretaceous. Along the east Mediterranean margin, some large E-W trending rifts formed, often with associated volcanism, e.g. southern Sirt and Abu Gharadig. Most researchers believe the oceanization of the eastern Mediterranean basin occurred at this time. Flexural subsidence affected the Moghrebian Atlas, including the Riffian-Tellian domain, where a thick flysch series was deposited. Two short-lived, but widespread and tectonically important, compressional or wrench-dominated events occurred during the Santonian and the latest Maastrichtian. From Morocco to Oman, most sedimentary basins were folded and inverted at these times. The Santonian event resulted from onset of the collision between the African-Arabian and Eurasian plates. New stress-fields favoured NE-SW extension along the African plate margin, generating or rejuvenating some rifts, e.g. in the Euphrates trough and in the Libya-Tunisia region (northwestern Sirt, Pelagian Sea). Rifting and magmatism continued in these regions into the Paleocene. During the Mesozoic, therefore, the northern margin of the African-Arabian plate registered both rifting resulting in the oceanization of the Tethys and rifting resulting from the initiation of the closure of the Tethys. The intraplate domain exhibited echoes of the tectonic events affecting the margin.