Impact of High Thermal Setting and Fluid Activities on Sandstone Compaction: A Case Study of the Baiyun Sag in the Pearl River Mouth Basin (Northern South China Sea)

Compaction is regarded as central to the reduction of reservoir physical properties. The thermal compaction process controlled by a basin’s heat flow and the static compaction caused by overload on rocks are both important factors controlling the compaction strength. However, porosity loss resulting from thermal and static compaction has not been distinguished. The Baiyun Sag in the Pearl River Mouth Basin in the northern part of the South China Sea with high heat flows and a variable geothermal gradient is an ideal setting for studying the characteristics and mechanisms of the thermal compaction process. The characteristics of compaction and the effect of thermal fluid activities on reservoir physical properties are carried out, based on the observation and identification of sandstone thin sections under a microscope, the measurement and simulation of the temperatures and trapping pressures of fluid inclusions, and the calculation of the compaction porosity loss as well. The result shows that the compaction mode of sandstone reservoirs in the Zhuhai Formation is dominated by static compaction in the LGR (the low geothermal gradient region), whereas the diagenetic process of the Zhuhai Formation in the MGR (the moderate geothermal gradient region) and HGR (the high geothermal gradient region) is affected not only by the static compaction effect but also by the thermal compaction effect caused by abnormal formation temperature and pressure conditions. The porosity loss caused by the thermal compaction ranges from 5.5% to 11.2% with an average of 7.9% and from 4.6% to 16.6% with an average of 10.2% in the MGR and HGR, respectively. The porosity loss caused by the static compaction ranges from 15.9% to 20.8% with an average of 19.4% and from 8.4% to 15.8% with an average of 12.8% in the MGR and HGR, respectively.

Static Compaction of Soils with Varying Clay Content

The use of compressed earth blocks (CEBs) is widespread in the field of earth construction. They present better mechanical performance than adobe and the equipment for their production is simple. Laboratory testing of compressed earth blocks requires large amounts of material. There are variations of unconfined strength testing procedures such as testing halves of the blocks with layers of mortar between them or testing whole blocks in diverse directions. This complicates the interpretation of test results as the shape factor and mortar characteristics influence the results significantly. Static compaction test can be used to produce cylindrical samples representative of CEBs. The water content of soil used for the production of CEBs is often determined in standard Proctor test while experimental data indicate that the optimum moisture content for static and dynamic compaction is different. The present article addresses the behavior of four soil mixes with varying clay content compacted statically with a constant rate of strain. Static compaction curves were compared with those obtained in standard Proctor test. For all the soil mixes the static optimum moisture content was found to correspond to the start of consolidation. The compaction curve presented no wet side of optimum in contrast to Proctor test. The energy needed to achieve a desired density by static compaction was analyzed for soils with varying clay contents. Static compaction was found to be more efficient than dynamic for clayey soils. An increase in water content was observed to help achieving higher densities at low pressures, which can improve the performance of manual CEB presses.