C-8 light hydrocarbons (LHs) were characterized for 23 crude oils from two different deposits by comprehensive two-dimensional gas chromatography. Dimethylcyclohexanes (DMCyC6) and trimethylcyclopentanes (TMCyC5) are the predominant compounds in C-8 LHs and geochemically significant. Used in tandem with deposition-diagnosed parameters such as pristane/phytane (Pr/Ph), dibenzothiophene/phenanthrene (DBT/P), C-22/C-21 tricyclic terpane (C-22/C21TT), C-29/C-30 hopane (C-29/C30H), and monoterpane ratios (MT1, MT2, and MTR), C8R ratio ((ltrans,4-+lcis,3-+1,1-) DMCyC6/(lcis,2trans,4cis- + lcis,2trans,3cis-) TMCyC5) can distinctly distinguish marine oils derived from source rocks deposited under anoxic conditions from the terrigenous oils originated from source rocks deposited in oxic/suboxic conditions, for their lower C8R (<5.0), Pr/Ph, and C-22/C21TT values, and higher DBT/P, C-29/C30H, and monoterpane ratio values. Followed after the classic isoheptane (P-1) and heptane (P-2) ratios, isooctane (O-1) and octane (O-2) ratios were proposed tentatively to define thermal maturity. The linear correlation between the O-1 with the P-1, and the O-2 with the P-2 values confirm their validity to assess and differentiate the thermal maturity according to the approximate same boundary values defined by that of P-1 and P-2 ratios.