Abstract
The Qiongdongnan Basin is a strongly overpressured basin with the maximum pressure coefficient (the ratio of the actual pore pressure versus hydrostatic pressure at the same depth) over 2.27. However, there exists a widespread low-overpressure interval between the strong overpressure intervals in the Yanan Sag of western basin. The mechanisms of the low-overpressure interval are not well understood. Three main approaches, pore pressure test data and well-log analysis, pressure prediction based on the relationship between the deviation of the velocity and the pressure coefficients, and numerical modeling, were employed to illustrate the distribution and evolution of the low-overpressure interval. And we analyzed and explained the phenomenon of the low-overpressure interval that is both underlain and overlain by high overpressure internal. The low-overpressure interval between the strong overpressure intervals can be identified and modelled by drilling data of P-wave sonic and the mud weight, and the numerical modeling using the PetroMod software. Results show that the low-overpressure interval is mainly composed of sandstone sediments. The porosities of sandstone in the low-overpressure interval primarily range from 15%-20%, and the permeabilities range from 10–100 md. Analysis of the geochemical parameters of C1, iC4/nC4, ΔR3, and numerical modeling shows that oil and gas migrated upward into the sandstone in the low-overpressure interval, and then migrated along the sandstone of low-overpressure interval into the Yacheng uplift. The low-overpressure both underlain and overlain by overpressure resulted from the fluids migrating along the sandstones in the low-overpressure interval into the Yacheng uplift since 1.9Ma. The mudstone in the strong overpressure interval is good cap overlain the sandstone of low-overpressure interval, therefore up-dip pinchouts or isolated sandstone in the low-overpressure interval locating the migration path of oil and gas are good plays for hydrocarbon exploration.
Highlights
Overpressure is an obvious geologic feature in some petroliferous basins and have been estimated using well data [1,2]
Based on the above analysis, the following conclusions can be drawn: 1. The low-overpressure interval between the strong overpressures can be identified by the Pwave sonic and the mud weight in that all overpressured mudstones have higher P-wave sonic compared with normally pressured mudstones at a given depth
The numerical modeling results using the PetroMod software show that there exists a lowoverpressure interval confirmed by the drilling and pressure prediction
Summary
Overpressure is an obvious geologic feature in some petroliferous basins and have been estimated using well data [1,2]. There exists a widespread strong overpressure in the middle-deep Formation (2900–5000 m) of the Qiongdongnan Basin, and the pressure coefficient can be up to 2.27 (the strong overpressure refers to that pressure coefficient is greater than 1.8) [3,4,5,6]. Overpressures (achieved by direct pore pressure measurements) in the Qiongdongnan Basin are associated with anomalously high porosity, compared with normally pressured sediments for a given depth [11]. The drilling analysis shows that there exists a composite configuration where the overpressure increases with depth at the upper interval, decreases at the middle interval, increases again at the lower interval [3,6,12]. The middle decreasing overpressure in this composite configuration is called the low-overpressure interval in this paper. The low-overpressure interval primarily lies in the Neogene strata of the Yanan sag [6]
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