DENSITY DISTRIBUTION OF CRUDE OIL IN THE ZHUYI DEPRESSION OF PEARL RIVER MOUTH BASIN AND CONTROL FACTORS

The Zhuyi Depression contributes the most of oil reserve in the Pearl River Mouth Basin. In order to confirm the distribution pattern and the control factors of different oil types in the target area, 261 crude oil density data from 79 wells are collected in this paper. Through careful study, it is observed that the density of oil changes in a wide range from light oil (140 samples) to medium oil (57 samples), with a minor of condensate (13 samples) and heavy oil (51 samples). The light to medium is distributed almost in all the oil fields, while the condensate is only distributed in the upper Zhujiang Formation of the Huizhou 26 oil field. Heavy oil is found in the upper Zhujiang Formation and the shallower reservoirs in the Enping Sag, Xijiang Sag and the Liuhua oil field of Huizhou Sag. Although the data covers a long depth range vertically, they are all less than 2000 meters in depth and the reservoir temperature is below 85 ℃. The condensate oil is the product of the shallow lacustrine-swamp source rock in a maturity stage and the density is obviously controlled by the type of source rock. Density of light-medium oil is controlled by both the oil types and maturity. Comparing to the shallow-semi-deep lacustrine oil, shallow lacustrine-swamp oil and the mixture of the semi-deep-deep lacustrine oil, the deep lacustrine oil has the heaviest density. Crude oil density declines with the increase in oil maturity. The density of heavy oil is mainly controlled by biodegradation and water washing. Both of the two factors can consume the lighter component of oil, and made it heavier. Through the analysis above, we can predict the oil types of main target formation according to the location and temperatures of the formation. The results could be used for the resource assessment and production prediction.