The presence of sedimentary rocks of G sequences in the LDX structure in Block 15-1/05, Cuu Long basin is a new discovery compared to previous studies on the geology and petroleum system in Block 15-1/05. The G sequences formations are temporarily named the Lac Da Vang formation, which is covered by Lac Da Nau formation (E sequences) and overlaid on granitoid and andesite - basalt intrusive of pre-Tertiary basement rocks. The studies on petrography, core and geophysical data of wells LDX-1X, LDX-2X and LDX-3X show that the G sedimentary rocks include interbeds of sandstone, siltstone, claystone/shale, and sedimentary rocks deposited in the alluvial fan, fan delta, braided fluvial and lacustrine environments. The reservoir quality of sedimentary rocks of G sequences is poor, due to the impact of the strong development of secondary minerals along with the burial compression process.

Key words: Lithology, sedimentary environment, secondary mineral, Lac Da Xanh structure, Cuu Long basin.