Hydrocarbon Fluid Inclusions in Halite From the Eocene Salt Deposit in Dongying Depression, Bohai Bay Basin, China: Evidence for Sealing Capacity of Rock Salt

Abstract

INTRODUCTION The Dongying depression, is a continental faulted basin, which is located at the southeastern part of the Jiyang subbasin in Bohai Bay Basin. The depression is filled with thick salt deposits in Eocene Sha-4 member of Shahejie Formation (Es4). Rock salt is known to many as nonporous, impermeable and ductile rock serving as the best seal for hydrocarbon accumulation. However, in this study, the presence of abundant hydrocarbon fluid inclusions (HCFI) in halite of Es4 in Dongying depression suggests otherwise. CHARACTERISTICS OF HYDROCARBON FLUID INCLUSIONS IN HALITE According to petrography and micro-fluorescence of fluid inclusions in halite collected from Es4, we identified that halite contains abundant HCFI, mainly distributed in Well DongFeng-3 and Well DongFeng-5. These HCFI can be divided into two generations: the first generation of HCFI mainly grew along the intragranular healed micro-fractures and the petroleum fluids within inclusions appear yellow or yellow-green in UV light, while the second generation of HCFI presents planar distribution within halite crystals, the petroleum fluids within inclusions appear light blue in UV light. In addition, the two generations of HCFI have a wide difference in amount, the first generation is major, while the second is minor. DISCUSSION ON SEALING CAPACITY OF ROCK SALT The presence of abundant HCFI in halite of Es4 is a direct evidence that rock salt has been permeable to hydrocarbon fluid (oil and gas) during specific geological period. Based on the systematic analysis (petrography, micro-fluorescence, and microthermometry) of HCFI in halite, combining with the reconstruction of burial history and thermal history, two stages of hydrocarbon fluid activities in rock salt of Dongying depression were determined: the first period occurred in Late Oligocene and another took place in Late Pliocene. Meanwhile, by analysis of the abundance of two generations of HCFI, the first period of hydrocarbon fluid activity was considered to be strong, indicating partial region of rock salt lost sealing capacity in Late Oligocene. In contrast, the second was relatively weak, the overall sealing capacity of rock salt in Late Pliocene is better than that in Late Oligocene.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015